input: Move synthetic web-builder util functions into common/.

This allows writing input-event related tests for the renderer side.

BUG=321457
R=jdduke@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/95283004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238082 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238082