Adds histogram timing for main-thread portions of streaming source
compilation. Also adds a histogram timer for capturing the amount of time
spent for off-thread parse / compile of streaming sources.

BUG=v8:5203

Change-Id: Ie9f16052205832a620cfbf266d3d66d3fe9d6c12
Reviewed-on: https://chromium-review.googlesource.com/797038Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49724}