OOPIF: Fix CapturePageInfo to only operate on local main frames.

This logic will need to move to the browser process to capture text
from frames that live in other processes.

BUG=346764
TEST=Navigate in an out-of-process iframe and wait 10 seconds.

Review URL: https://codereview.chromium.org/645803002

Cr-Commit-Position: refs/heads/master@{#299138}
1 file changed