The benefits: Android lint can find resources that don't exist in all
resolutions, it's clearer what resolution these images are, and the drawable
folder is neater as well.

The build output after this change is identical, thanks to recent build changes
(https://codereview.chromium.org/321463002) as well as the method
MoveImagesToNonMdpiFolders() in package_resources.py.

BUG=289843
R=dtrainor@chromium.org, tedchoc@chromium.org

Review URL: https://codereview.chromium.org/340913003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278745 0039d316-1c4b-4281-b951-d872f2087c98