Make mojo_benchmark understand the output it is producing.

This patch:
 - refactors the logic in mojo_benchmark so that obtaining the output
   produced by `benchmark.mojo` is separated from its analysis and
   presentation
 - makes `mojo_benchmark` parse the measurements results, as opposed to
   just streaming the output produced by `benchmark.mojo`.

This is a prerequisite for producing the chart data to be uploaded to
the performance dashboard, see domokit/mojo#474. As a bonus, we now
notice when measurement result is missing and return non-zero exit code.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1392173006 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 0acce94790935a282c3461c3fca2787e61bd2233