This allows benchmarking without --single-process and correctly
gathering --runtime-call-stats numbers.

Add optional parameter to %GetAndResetRuntimeCallStats

-  Without any parameter, it returns a string with the runtime call
   statistics (as before).
-  With one string parameter, it appends the statistics to the file with
   that file name.
-  With one integer parameter (which must be 1=stdout or 2=stderr), it
   prints the statistics to the output with of that file descriptor.

The injected script is automatically generated.
Also, callstats.py does not have a hardwired DEFAULT_SITES anymore.

R=cbruni@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/1966193002
Cr-Commit-Position: refs/heads/master@{#36172}