Temporarily disable sync to diagnose crashes

[kjozwiak]

Description

We'll be temporarily disabling sync to diagnose several crashes and improve general stability.

Steps to Reproduce

• Test Plan: Append --disable-sync swtich to disable sync temporarily brave-core#4565

Actual result:

Brave is currently crashing due to several crashes which is affect user experience and stability.

Expected result:

Temporarily disabling sync so we can diagnose the current crashes and improve stability.

Reproduces how often:

We don't know how many users are currently affected but we've received numerous reports from various users on different social network platforms including Brave Community.

Brave version (brave://version info)

Brave	1.3.113 Chromium: 80.0.3987.87 (Official Build) (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}

Brave	1.3.114 Chromium: 80.0.3987.87 (Official Build) (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the dev channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? N/A
• Does the issue resolve itself when disabling Brave Rewards? N/A
• Is the issue reproducible on the latest version of Chrome? N/A

Miscellaneous Information:

CCing @bbondy @rebron @anthonypkeane @ jsecretan @brave/legacy_qa

[srirambv]

Verification passed on

Brave	1.3.115 Chromium: 80.0.3987.87 (Official Build) (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}
OS	Linux

• Verified test plan from Append --disable-sync swtich to disable sync temporarily brave-core#4565
• Verified Sync is disabled via command line flag found in brave://version
  
• Verified Sync is greyed out in hamburger menu

Verification passed on

Brave	1.3.115 Chromium: 80.0.3987.87 (Official Build) (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}

• Verified test plan from Append --disable-sync swtich to disable sync temporarily brave-core#4565
• Verified Sync is disabled via command line flag found in brave://version
  
• Verified Sync is greyed out in hamburger menu
• Verified visiting brave://sync shows invalid page error
• Verified Extension: Brave Sync process isn't listed in task manager

Verified passed with

Brave	1.3.115 Chromium: 80.0.3987.87 (Official Build) (64-bit)
Revision	449cb163497b70dbf98d389f54e38e85d4c59b43-refs/branch-heads/3987@{#801}
OS	macOS Version 10.14.6 (Build 18G103)

• Verified test plan from Append --disable-sync swtich to disable sync temporarily brave-core#4565
• Verified Sync is disabled via command line flag found in brave://version

• Verified visiting brave://sync shows invalid page error
• Verified Extension: Brave Sync process isn't listed in task manager

• Encountered sync still shows as disabled in hamburger menu #8158, sync still shows as an option in settings #8159

[kjozwiak]

Please see #8128 & #8108 for more context re: the reasons for temporarily disabling Sync.