Pepper: Add IPC enum max checking for PP_FileSystemType, PP_FileType.

BUG=194304

Review URL: https://codereview.chromium.org/64913006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234381 0039d316-1c4b-4281-b951-d872f2087c98
Move scheme/method prerender checks to the throttle.

This moves the logic into the prerender/ directory. The requests also get
cancelled via a ResourceController rather than calling Cancel() on the
URLRequest directly.

This also has the side effect of always performing the HandleExternalProtocol
check before the scheme check, so Prerender.SchemeCancelReason should report
"external protocol" vs. "unknown" more consistently. (We currently report the
former on redirect and the latter when navigating directly.)

BUG=316634
TEST=no behavior change (apart from histograms)

Review URL: https://codereview.chromium.org/66193008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234382 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234382