Move webp include rule to platform/DEPS.

The source files which include webp headers have moved to platform/.
The corresponding DEPS rule should move along with them.

This fixes a few check-blink-deps errors.

BUG=

Review URL: https://codereview.chromium.org/98233008

git-svn-id: svn://svn.chromium.org/blink/trunk@163550 bbb929c8-8fbe-4397-9dbb-9b2b20218538