Relax self-referential frame checks for non-initial navigations.

Relax self-referential frame checks for non-initial navigations.

The improved self-referential frame blocking in r450728 broke sites
that rely on constructing frame hierarchies where frames are loaded
via POSTs with the same URLs.  To fix this, and potentially other
breakage from the tightened check, only enforce self-referential URL
checks for the initial load in a frame.  There's no way for a frame's
first committed URL to be a POST, so this fix handles POST requests
automatically.  It also more closely matches the old Blink check (that
was moved to browser process in r450728), which was only enforced on
initial navigation during frame creation and on changes to an iframe's
"src" attribute (the latter bit is lost with this more conservative
check).

BUG=710008
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

[alexmos, 2017-04-13 21:22:49]

The CQ bit was checked by alexmos@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-13 21:23:31]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...