- Implement RunMicrotasks in CSA to prevent a potentially large number
  of jumps between C++ and JS code while consuming te queue. Appears to
  provide a ~60% speedup in microtask-heavy code, which from limited
  testing appears to scale linearly.

  The code-stub microtask pump bails out to the old C++ microtask pump
  if it encounters a CallHandlerInfo microtask, and remains in C++ for
  the remainder of the queue (returning to the JS/stub implementation
  after the bailed out queue is exhausted).

- Add a variation of JSEntryStub which enters the new RunMicrotasks code
  stub.

- Add a new RunMicrotasks helper to Execution, which uses the
  RunMicrotasks entry stub.

Bug: 
Change-Id: I4667d4dd633d24455ea5d7cef239da0af1a7365e
Reviewed-on: https://chromium-review.googlesource.com/650486
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49842}