Refactored pixel tests' cloud storage code and options into base classes.

Refactoring only. No functionality changes.

Tested locally, including forcing a failure and testing error image
upload path.

BUG=330774
TBR=bajones@chromium.org

Review URL: https://codereview.chromium.org/113633010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242636 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242636