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Mainly, this cl add the two methods to clear cache.
1) GDataCache::ClearAllOnUIThread():
- remove all the files in the cache directory.
- re-create the |metadata_| instance.
2) GDataSystemService::ClearCacheAndReset():
- unmount Drive
- cancel all the running tasks.
- call GDataCache::ClearAllOnUIThread()
- re-mount Drive

BUG=135197
TEST=out/Debug/unit_tests:GData* and out/Debug/browser_tests:GData* passes.

Review URL: https://chromiumcodereview.appspot.com/10823125

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149914 0039d316-1c4b-4281-b951-d872f2087c98