Reland "Decode GIF image frames on demand".

Reland "Decode GIF image frames on demand".

Change over the original patch: workaround const_reverse_iterator issue of
Android compiler.

Previously GIFImageDecoder::frameBufferAtIndex() decodes all frames from
the last decoded frame to the requested frame. This causes delays when
skipping to specific frames.

With this change, GIFImageDecoder decodes only necessary frames based on
previous frames' disposal methods. Added ImageDecoder::findRequiredPreviousFrame()
to find the required previous frame based on the disposal methods.

Also changed the behavior of GIFImageDecoder::clearFrameBufferCache() (moved to
ImageDecoder because it can be shared between multi-frame decoders) because
the original semantics no longer apply with on-demand frame decoding.
Instead of clearing before the given frame, now clear from the whole frame buffer
cache, preserving the frames that are likely to be used soon.

BUG=238242
TEST=ImageDecoderTest.requiredPreviousFrameIndex*, ImageDecoderTest.clearCacheExceptFrame*, GIFImageDecoderTest.updateRequiredPreviousFrameAfterFirstDecode, GIFImageDecoderTest.randomFrameDecode, GIFImageDecoderTest.randomDecodeAfterClearFrameBufferCache, GIFImageDecoderTest.resumePartialDecodeAfterClearFrameBufferCache
TBR=abarth@chromium.org, pkasting@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=151564

[Xianzhu, 2013-05-31 18:08:04]

Committed patchset #1 manually as r151564 (presubmit successful).

[Peter Kasting, 2013-05-31 20:31:59]

Was there a change since the original landing?

[Xianzhu, 2013-05-31 21:00:09]

On 2013/05/31 20:31:59, Peter Kasting wrote:
> Was there a change since the original landing?

I should have uploaded the original patch first.

The change is a workaround for Android compiler in GIFImageDecoder.cpp,
mentioned in the description:

-    for (Vector<size_t>::const_reverse_iterator iter = framesToDecode.rbegin();
iter != framesToDecode.rend(); ++iter) {
+    Vector<size_t>::const_reverse_iterator rend = framesToDecode.rend();
+    for (Vector<size_t>::const_reverse_iterator iter = framesToDecode.rbegin();
iter != rend; ++iter) {

[Peter Kasting, 2013-05-31 21:01:55]

On 2013/05/31 21:00:09, Xianzhu wrote:
> On 2013/05/31 20:31:59, Peter Kasting wrote:
> > Was there a change since the original landing?
> 
> I should have uploaded the original patch first.
> 
> The change is a workaround for Android compiler in GIFImageDecoder.cpp,
> mentioned in the description:
> 
> -    for (Vector<size_t>::const_reverse_iterator iter =
framesToDecode.rbegin();
> iter != framesToDecode.rend(); ++iter) {
> +    Vector<size_t>::const_reverse_iterator rend = framesToDecode.rend();
> +    for (Vector<size_t>::const_reverse_iterator iter =
framesToDecode.rbegin();
> iter != rend; ++iter) {

Ugh, that sucks.  What was the Android compiler's issue?  Can we put a comment
in here about why we wrtoe the code this way, and how someone would know it's
safe to change it back?