Remove duplicated headers from the platform/ directory where the implementation file includes the same dependencies as the corresponding primary header. 

BUG=317231

Review URL: https://codereview.chromium.org/64643010

git-svn-id: svn://svn.chromium.org/blink/trunk@161712 bbb929c8-8fbe-4397-9dbb-9b2b20218538