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This fixes flakiness in a variety of browser tests on Android caused
by nested calls to Java_NestedSystemMessageHandler_dispatchOneMessage().
This method calls into looper code that is not reentrant safe which
results in crashes. This change refactors the MessagePumpUI &
NestedMessagePumpAndroid logic so that work can be dispatched without
the looper code on the call stack. The code detects whether work events
are being dispatched while inside the looper and defers that work until
the Java_NestedSystemMessageHandler_dispatchOneMessage() call returns.
This allows us to avoid making reentrant calls into the looper while
still allowing nested RunLoops to work properly.

- Splits MessagePumpUI::OnDelayedLooperCallback() and
  MessagePumpUI::OnNonDelayedLooperCallback() into 2 pieces so the
  dispatching of work can be separated from the looper callback.
- Added deferred work tracking logic to NestedMessagePumpAndroid so
  that it can keep track of what events occurred while the looper was
  running and dispatch work once the looper returns.
- Added logic to save/restore the delegate and quit status so that
  quitting one Run() level does not quit all levels. This makes the
  code more consistent with other MessagePump implementations.
- Enable tests on Android that were previously disabled because of
  flakiness caused by crashing in the looper.


Bug: 1110497, 1129592
Change-Id: I4cfb8804275045b6fb4ab2b0d89cdc20fdb8ea4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511790Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Aaron Colwell <acolwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827283}