Fixes application crashes when calculating the winning page classification category

[tmancey]

Resolves brave/brave-browser#7866

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protection-Mode
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

See brave/brave-browser#7866

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[emerick]

LGTM

[SergeyZhukovsky]

see comments

[SergeyZhukovsky]

Are you sure we don't need a check on a null pointer for user_model_? As that is the reason of the crash. Maybe we should fail with empty winning_categories?

[SergeyZhukovsky]

We've discussed this on a call, the place should never be reached when the pointer is null. Let's
apply the fix and reopen the issue if we still see it.

[SergeyZhukovsky]

++

[tmancey]

CI failed on Linux, restarting CI for Linux

[tmancey]

CI failing on Linux for unrelated issues, merging

[NejcZdovc]

checked CI with @tmancey, linux looks unrelated

[kjozwiak]

Reproduced the original issue on macOS 10.15.3 x64 using the following build:

Brave	1.7.26 Chromium: 80.0.3987.122 (Official Build) nightly (64-bit)
Revision	cf72c4c4f7db75bc3da689cd76513962d31c7b52-refs/branch-heads/3987@{#943}
OS	macOS Version 10.15.3 (Build 19D76)

Using the STR from #4770 (comment), ran into the following crash/error:

[0302/112312.477864:WARNING:process_memory_mac.cc(93)] mach_vm_read(0x7ffee9edb000, 0x2000): (os/kern) invalid address (1)
[0302/112312.560187:WARNING:system_snapshot_mac.cc(42)] sysctlbyname kern.nx: No such file or directory (2)

• https://stats.brave.com/dashboard#crash/5e5d31254a16f2006a000abd
• https://stats.brave.com/dashboard#crash/5e5d32f1da2b9d00672c38d8

Verification PASSED on macOS 10.15.3 x64 using the following build:

Brave	1.7.32 Chromium: 80.0.3987.122 (Official Build) nightly (64-bit)
Revision	cf72c4c4f7db75bc3da689cd76513962d31c7b52-refs/branch-heads/3987@{#943}
OS	macOS Version 10.15.3 (Build 19D76)

• Went through the STR mentioned via Fixes application crashes when calculating the winning page classification category #4770 (comment) and ensured that the above crash doesn't occur and the ad is being displayed.