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This CL fixes three corner cases:
1) This CL sets the "WebAPK update" as having succeeded if:
  - the previous WebAPK update failed
  AND
  - A WebAPK update is no longer needed
  Doing this is important because the is-webapk-update-needed check is
  done much more frequently if the "previous WebAPK update failed."
2) This CL ignores whether the "WebAPK update" has previously succeeded if
   there have been no update attempts.
   The goal is to wait at least WebApkUpdateManager#FULL_CHECK_UPDATE_INTERVAL
   since the WebAPK's first launch before checking for updates.
3) This CL sets the "WebAPK update" as having failed if Chrome is killed
  after the time that the WebAPK update is requested but before
  WebApkUpdateManager#onBuiltWebApk() is called.

BUG=639536
TEST=WebApkManagerTest.*
R=dominickn, mlamouri
TBR=tedchoc (For new dependencies to chrome_junit_tests in BUILD.gn)

Review-Url: https://chromiumcodereview.appspot.com/2430773002
Cr-Commit-Position: refs/heads/master@{#426868}