Rewrite platform headers in public/web/ to public/platform/...

Includes of this style are more consistent with the rest of the project and
easier to verify with DEPS. For consistency, includes in public/web/ of other
headers in public/web/ are rewritten to list the public/web/ portion as well.

BUG=350097

Review URL: https://codereview.chromium.org/189703002

git-svn-id: svn://svn.chromium.org/blink/trunk@169597 bbb929c8-8fbe-4397-9dbb-9b2b20218538