Invalid memory access after WaitableEvent timed out

[darkdh]

reopen #6169
and extra crash reports
https://stats.brave.com/dashboard#crash/5d9ce39d13e758001f81da76
https://stats.brave.com/dashboard#crash/5d9cadeb997f1a001fb9b5fa

STR:

1. Open Brave with --enable-logging
2. Establish sync chain
3. Open chrome://inspect#extensions, make sure you see "sync-debug" message="success"
4. Make OS go offline
5. Monitor chrome_debug.log in user profile or console log from terminal until you see [...WARNING:syncer.cc(37)] WaitableEvent timed out (About 3-4 mins)
6. Make OS go online after seeing the step 5 message

[GeetaSarvadnya]

Verification passed on

Brave	0.69.134 Chromium: 77.0.3865.120 (Official Build) (64-bit)
Revision	416d6d8013e9adb6dd33b0c12e7614ff403d1a94-refs/branch-heads/3865@{#884}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan from Remove fail-safe WaitableEvent::TimedWait() because brave-core#3636

Step2 : "sync-debug" message="success"

Step3: when OS is offline "Uncaught (in promise) TypeError: Failed to fetch",

Step 5: When OS in online "fetch-sync-records" category_names=["BOOKMARKS"]...

Verification passed on

Brave	0.69.135 Chromium: 77.0.3865.120 (Official Build) (64-bit)
Revision	416d6d8013e9adb6dd33b0c12e7614ff403d1a94-refs/branch-heads/3865@{#884}
OS	Ubuntu 18.04 LTS

• Verified the test plan from Remove fail-safe WaitableEvent::TimedWait() because brave-core#3636

Step2 : "sync-debug" message="success"

Step3: when OS is offline "Uncaught (in promise) TypeError: Failed to fetch",
Step 5: When OS in online "fetch-sync-records" category_names=["BOOKMARKS"]...