Animated WebP: Optimize decoding in case of seeking

In case of WebP, it can be inferred whether an animation frame contains
alpha without decoding the frame.
WebP also has a mode where a frame overwrites the corresponding
rectangle on the previous frame.
These two were added to Chromium here:
https://codereview.chromium.org/22802020/

So, we optimize findRequiredPreviousFrame() for cases when a frame is
known to be opaque or it overwrites pixels on the previous frame.

BUG=229641

Review URL: https://chromiumcodereview.appspot.com/23068027

git-svn-id: svn://svn.chromium.org/blink/trunk@156723 bbb929c8-8fbe-4397-9dbb-9b2b20218538