Rename RasterDecoderContextState to DecoderContextState

Also convert RasterDecoderContextState from struct to class.

TODO:
 * rename raster_decoder_context_state.h to decoder_context_state.h

Bug: 919865
Change-Id: I77cb8d369c1e0c736434da6bbb144c863d9ec3f6
Reviewed-on: https://chromium-review.googlesource.com/c/1426060
Commit-Queue: Peng Huang <penghuang@chromium.org>
Auto-Submit: Peng Huang <penghuang@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625194}
31 files changed