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Previously we disabled POST requests from being snapshotted by OPC.
However, this inadvertently prevented SnapshotController from marking
a WebContents as snapshottable, so any POST requests would be unable to
complete - they would hang in downloading state until Chrome exited.
After that, the background loader would take over and error out because
it does not support non-GET requests and would use a GET HTTP request
for a POST resource which typically fails or redirects to a login page
of some kind.

Additionally, extends NavigationSimulator to use the method from
SetMethod when committing navigations, rather than the currently-hardcoded
"GET" method.

Bug: 859403
Change-Id: Ic10dd0b088da9e3ebf2a42e3cca6096eea449d16
Reviewed-on: https://chromium-review.googlesource.com/1125031Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573516}