gif2webp: Handle frames with missing  graphic control extension
gif2webp: Handle frames with missing  graphic control extension

According to the GIF spec (http://www.w3.org/Graphics/GIF/spec-gif89a.txt),
this block is optional, and its scope is only the first graphic rendering block
that follows.

The spec doesn't mention what default values of frame dimensions, offsets,
duration and transparent index to use in such a case, though. So, we use the
defaults used by GIF reader in Chromium:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageReader.h&l=186

Change-Id: Iecc6967847192483770e85ac15fe2835cd01ce7b

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