Revert of Allow frames to be free'd even for static images (https://codereview.chromium.org/136503002/)

Reason for revert:
May have caused issue 350653.

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=165127

TBR=hclam@chromium.org,pkasting@chromium.org,senorblanco@chromium.org,jochen@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=302851

Review URL: https://codereview.chromium.org/201793002

git-svn-id: svn://svn.chromium.org/blink/trunk@169347 bbb929c8-8fbe-4397-9dbb-9b2b20218538