Use more forward declarations in gpu code

Some of the files were relying on gles2_cmd_decoder.h to include
glx::GLContext and glx::Size for them, so extra includes were added
there.

BUG=None

Review URL: https://codereview.chromium.org/1201193002

Cr-Commit-Position: refs/heads/master@{#336128}
8 files changed