Port 52ff3ae4

Original Commit Message:

    - 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.

R=caitp@igalia.com, joransiu@ca.ibm.com, jbarboza@ca.ibm.com
BUG=
LOG=N

Change-Id: Ifa15ca19312bb92758e82d19c3e3fc0a8b908d82
Reviewed-on: https://chromium-review.googlesource.com/820197Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#50018}