[AW] Refactor CrashInfo to have all crash keys

Refactor CrashInfo common class to have all crash key-value pairs
extracted from the crash minidump report instead of app package name
and variations only. This will be useful to fix some bugs in the crash
UI.

No method to access variations is provided since it's not used any
where.

Improve test coverage for CrashInfo class:
- Move merge tests from WebViewCrashInfoCollectorTest to CrashInfoTest.
- Make mismatch assert message more readable by implementing
  CrashInfoEqualityMatcher#describeMismatch and reuse the matcher class
  in all tests.
- Add unit tests for parsing and serializing JSON strings.

Bug: 1077783,1073934
Fixed: 1022346,1075648
Test: out/Debug/bin/run_webview_instrumentation_test_apk
Change-Id: Ic65ced7c11b8eb81706d748e9710fef0bb76057a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2196125
Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769684}