Rather than having a single script compilation timer, split it into
multiple timers depending on the state of the (blink-owned) code cache
and (v8-owned) complation cache. This is intended to replace both the
script compilation time timer, and the compilation heuristic enum.

Also keep track of why blink might not want us to produce (or consume) a
code cache, and split the compilation timer on this as well.

Note, there is currently no timer for streaming sources, so these won't
show up in the histograms.

Bug: chromium:582873
Bug: chromium:769203
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ia32fff044f919e20e3cec73329e62e01e421b72a
Reviewed-on: https://chromium-review.googlesource.com/746922
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49065}