Remove duplication of encoded image data

Now there are at least two encoded image data: one is in Image/ImageResource as
SharedBuffer and the other is in DeferredImageDecoder as SkRWBuffer. This CL
removes former when possible (non-icon bitmaps), and generate the SharedBuffer
from the SkRWBuffer if needed.

Design Doc: https://docs.google.com/document/d/1v0yTAZ6wkqX2U_M6BNIGUJpM1s0TIw1VsqpxoL7aciY/edit?usp=sharing

BUG=618623
TEST=blink_platform_unittests --gtest_filter=BitmapImageTest.*:ImageDecoderTest.*

Review-Url: https://codereview.chromium.org/2054643003
Cr-Commit-Position: refs/heads/master@{#406238}
23 files changed