Remove useless class and struct forward declarations, and add
missing ones in media header files.

BUG=662195
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2533273002
Cr-Commit-Position: refs/heads/master@{#435198}