blink: Cleanup class/struct forward declarations

Remove useless class and struct forward declarations, and add
missing ones in blink header files. This is not the first time
such a cleanup is done for blink, but it should be the last as
we have now an up-and-running presubmit check.

BUG=662195
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2542203004
Cr-Commit-Position: refs/heads/master@{#436168}
140 files changed