[Android WebView] Fix AwQuotaManagerBridgeTest tests

This change makes the tests resilient to the situation
when AppCache is non empty in the beginning of the test.

BUG=324714,171765
R=mkosiba@chromium.org

Review URL: https://codereview.chromium.org/99023002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238110 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238110