Remove UnusedParam.h as most of its use is not not needed.

Chromium and Blink sets -Wno-unused-parameter intentionally, so no need to silence related warnings using the UNUSED_PARAM macro. 

Introduce ALLOW_UNUSED for Blink to mark local variables that might be unused to silence -Wunused-variable. The same macro exists on the chromium side with the same definition.

Replace UNUSED_PARAM with the newly introduced ALLOW_UNUSED where needed.

TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/34273002

git-svn-id: svn://svn.chromium.org/blink/trunk@163467 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Test for https://src.chromium.org/viewvc/blink?revision=163447&view=revision

This covers the case where an iframe initially loads about:blank, navigates
again to about:blank, then does a real navigation. We were incorrectly creating
a new history entry for the final navigation. The entire operation should
constitute a single entry.

BUG=323574

Review URL: https://codereview.chromium.org/106983008

git-svn-id: svn://svn.chromium.org/blink/trunk@163468 bbb929c8-8fbe-4397-9dbb-9b2b20218538