Map delegated software bitmaps only when they're about to be read from.

      If frames are delegated from one renderer through another into the browser, the intermediate renderer may not be able to map the shared memory bitmaps. However, it also won't need to read from the bitmaps, so the resource provider only needs to map the bitmaps when trying to read from them.

BUG=

Review URL: https://codereview.chromium.org/142403005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250404 0039d316-1c4b-4281-b951-d872f2087c98