Implement AggregatableHistogramTimer and use it to measure how much time

Implement AggregatableHistogramTimer and use it to measure how much time
of running a script is really spent in compilation. That is, sum up the
total time spent compiling (parsing + compile proper) within a run call
as seen through the API.

@jochen: So many questions:
- Is it ok to re-use V8.CompileLazy?
  This measures something a little different.
- clang-format does funny things to the huge macro definitions.
  I accepted clang-format changes for all code, but reverted for
  the #define orgies in counters.h. ok?
- Am I measuring the right thing. That is, are Aggregat[ing|ed]TimerScope
  in the right place?

I'll fiddle a bit more with this to see if it does the right thing. Would
be happy if you could still review now-ish.

BUG=

Review URL: https://codereview.chromium.org/790413004

Cr-Commit-Position: refs/heads/master@{#26226}