Fix infinite loading for small images in Files.app.

When images are small and the same as thumbnails, then the onload callback is not called. This patch fixes this by explicitely clearing the src attribute before any load.

TEST=Follow the bug's repro steps on Drive. The image should load fast.
BUG=248235

Review URL: https://chromiumcodereview.appspot.com/16124013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@205782 0039d316-1c4b-4281-b951-d872f2087c98