Hide sync option in settings if it's disabled

[simonhong]

fix brave/brave-browser#8159

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#8159

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.

[bsclifton]

Changes LGTM (pending addressing feedback by @darkdh)

For others reading this, user would need to enable flag via brave://flags

That would set the isSyncEnabled to true. I verified this works great locally 😄 👍

[bsclifton]

++

[simonhong]

Got unrelated build error on Win64.
https://ci.brave.com/job/brave-browser-build-pr/job/hide_sync_menu_from_settings/4/execution/node/287/log/

09:38:25  In file included from ../../brave/browser/tor/tor_profile_service_impl.cc:14:
09:38:25  ../..\brave/browser/brave_browser_process_impl.h(14,10): fatal error: 'brave/components/brave_referrals/buildflags/buildflags.h' file not found
09:38:25  #include "brave/components/brave_referrals/buildflags/buildflags.h"
09:38:25           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
09:38:25  1 error generated.

Started to rebuild only on Win64.

[simonhong]

CI success on all platform.

[LaurenWags]

Verified using

Brave	1.6.30 Chromium: 80.0.3987.106 (Official Build) nightly (64-bit)
Revision	f68069574609230cf9b635cd784cfb1bf81bb53a-refs/branch-heads/3987@{#882}
OS	macOS Version 10.14.6 (Build 18G103)

Verified STR from brave/brave-browser#8159

With sync flag set as Default/Disabled:

With sync flag set as Enabled: