Implement new willStartMainResourceRequest method of blink::WebApplicationCacheHost

This separates the frame-walking code from the appcache-setup code, allowing
the next Blink patch in this sequence to handle frame-walking on the Blink side.

BUG=343272
TBR=kinuko@chromium.org

Review URL: https://codereview.chromium.org/161573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251187 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251187