Add options to GPU pixel test to use cloud storage for reference and error images.
Add options to GPU pixel test to use cloud storage for reference and error images.

In order to run the pixel tests in isolated mode it's necessary to
stop storing the reference and generated images on the local disk.
This CL adds command line options to the test to optionally store and
retrieve reference images from cloud storage, and to upload any error
images to cloud storage directly, instead of using the separate
archive_gpu_pixel_test_results step.

Once this lands, the GPU recipe will be updated to switch to the new
command line arguments, at which point the old on-disk storage code
path will be deleted.

As part of this:

 - Added an Exists() method to Telemetry's cloud_storage for
   convenience, and added an option to make the uploaded file publicly
   visible (required for the GPU pixel tests' web app).

 - Added Bitmap.WritePngToFile to supplement the method which takes a
   file path.

BUG=330053
TEST=tested manually locally and verified upload, download, and error paths all work

Review URL: https://codereview.chromium.org/106523006

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@242289 0039d316-1c4b-4281-b951-d872f2087c98

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