Allow frames to be free'd even for static images

r151702 added limit for pruning in ImageDecoded::clearCacheExceptFrame,
which would disallow pruning single-frame/static images altogether.

Allow such images to be pruned in their entirety by dropping the
condition for <= 1 frames - the for-loop will handle the kNotFound case.

(Also related is r156651, which re-added the destroyAll flag.)

Modify the ImageFrameGeneratorTest.frameHasAlpha unittest to use frame 0
instead of frame 1, to avoid clearing frame 1 (MockImageDecoder
maps all indices to frame 0.)

R=hclam@chromium.org,pkasting@chromium.org,senorblanco@chromium.org
BUG=302851

Review URL: https://codereview.chromium.org/136503002

git-svn-id: svn://svn.chromium.org/blink/trunk@165127 bbb929c8-8fbe-4397-9dbb-9b2b20218538