Refactor gn versionCode logic and add tests

Refactor existing chrome for android and webview version code
logic from build/config/android/chrome_version.gni to a python
script and add unit tests (in PRESUBMIT).

Bug: 938138, 945039
Test: Add unit tests in PRESUBMIT; manually verify versionCode
Test: values in manifest.xml using Android Studio apk analyzer
Change-Id: I6537c8f2d4d1f6ee9a18c43f253b69de0afe674a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501459
Commit-Queue: Drew Stonebraker <stonebraker@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Tao Bai <michaelbai@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645096}
13 files changed