Changes from first reland attempt at https://crrev.com/c/1011287:
- fix an additional source of flakiness in ChromeOS login tests

Changes from original attempt at https://crrev.com/c/999182:
- fix flakiness in two additional ChromeOS login tests
- fix CSP WPT tests to not depend on ordering between iframe's onload
  and postMessage - see https://crbug.com/832319.

Previously, we sent the IPC to forward a cross-process postMessage
immediately.  This caused a behavioral difference from the
same-process case where the postMessage is always scheduled.  Namely,
in a scenario like this:

  frame.postMessage(...);
  doSomethingThatSendsIPCsToFrame(frame);

the IPCs from JS following the postMessage would've been ordered
incorrectly, causing |frame| to see their side effects after the
postMessage dispatch in the cross-process case, whereas they would be
seen before the postMessage dispatch in the same-process case.  One
example of this is frame.focus(), and another is a frame element
onload event (dispatched via FrameHostMsg_DispatchLoad) arriving after
a postMessage dispatched from an inline script while the frame was
still loading.

To resolve these ordering concerns, this CL changes cross-process
postMessage to do a PostTask on the sender side before sending the
message to the browser process.  This improves the current state of
the world, but does not yet achieve a perfect match for the IPC
ordering in the same-process case - see discussion on the bug.

Bug: 828529
Tbr: dcheng@chromium.org
Change-Id: If2cc6591db31471adff0d84ec0b689905e21cdf1
Reviewed-on: https://chromium-review.googlesource.com/999182
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Original-Original-Commit-Position: refs/heads/master@{#550284}
Reviewed-on: https://chromium-review.googlesource.com/1011287
Cr-Original-Commit-Position: refs/heads/master@{#550621}
Reviewed-on: https://chromium-review.googlesource.com/1012472
Cr-Commit-Position: refs/heads/master@{#550769}