Explicitly wait for sad frames in two ChildFrameCrashMetrics tests.

Previously, the KilledWhileVisible and KilledWhileHiddenThenShown
tests relied on sad frame metrics being logged synchronously after a
process kill or after a WebContents visibility.  Make these tests
more robust by explicitly waiting for all sad frames to be shown before
checking metrics.  This is an attempt to deflake these tests.

Additionally, add some explicit expectations around subframe visibility
on a shown/hidden a WebContents, to aid debugging in case these tests
are still flaky.

Bug: 1097060
Change-Id: I05567e9c76f94f32c0f2bbabf47fc2cfa2b013e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2509003
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822772}