Refactor EventBuffer into TestEventSink.

- Got rid of EventProcessor inheritance to reduce the dependency.
- Moved closer to the beginning as a preparation to use this
  in EventRewriterTest in following CLs.
- Refactored PopEvents taking a buffer as argument into TakeEvents
  returning the result directly.
- Replaced some EXPECT_* macros by ASSERT_* macros specifically
  if the following assertions assume the assertion passes.
- Tweaking empty lines to make better grouping of test blocks.
- Use Event::AsKeyEvent() instead of static_cast for better
  maintenance.

BUG=1440147
TEST=Tryjob

Change-Id: I37ad16602a80a2f39349bfcec86de672b0f890be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4957499
Reviewed-by: David Padlipsky <dpad@google.com>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1213975}