Remove remaining useless class and struct forward declarations from
Chromium (excluding third_party directory) using a homemade script
available in bug 662195 comments. An up-and-running presubmit check
is now available and should prevent new useless forward declarations
to come up.

BUG=662195
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_nacl_sdk

Review-Url: https://codereview.chromium.org/2554863002
Cr-Commit-Position: refs/heads/master@{#436883}