mojo_benchmark: improve recording the chart data for perf dashboard.

mojo_benchmark: improve recording the chart data for perf dashboard.

This patch:
 - adds the neccessary metadata to `chart_data` being recorded
 - records individual `chart_data` for individual benchmarks (so that
   all measurements won't be tracked on one collective chart)
 - fixes an import error introduced in
   64736fae560422778d97e5e747458e34697cd24e

R=etiennej@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/7615d79003f86ad1958e4a0c2d9a613d01a50569

[ppi, 2015-10-23 08:54:58]

ppi@chromium.org changed reviewers:
+ etiennej@chromium.org

[ppi, 2015-10-23 08:54:59]

Hi Etienne,

ptal.

[etiennej, 2015-10-23 10:02:13]

https://codereview.chromium.org/1412113003/diff/1/mojo/devtools/common/mojo_b...
File mojo/devtools/common/mojo_benchmark (right):

https://codereview.chromium.org/1412113003/diff/1/mojo/devtools/common/mojo_b...
mojo/devtools/common/mojo_benchmark:247:
script_args.chart_data_output_file.write(chart_data_recorder.get_json())
Do you want to add a newline at the end of the json string?

[ppi, 2015-10-23 10:59:13]

https://codereview.chromium.org/1412113003/diff/1/mojo/devtools/common/mojo_b...
File mojo/devtools/common/mojo_benchmark (right):

https://codereview.chromium.org/1412113003/diff/1/mojo/devtools/common/mojo_b...
mojo/devtools/common/mojo_benchmark:247:
script_args.chart_data_output_file.write(chart_data_recorder.get_json())
On 2015/10/23 10:02:12, etiennej wrote:
> Do you want to add a newline at the end of the json string?

Done.

[etiennej, 2015-10-23 11:00:07]

Thanks, lgtm

[ppi, 2015-10-23 11:04:45]

Committed patchset #2 (id:20001) manually as
7615d79003f86ad1958e4a0c2d9a613d01a50569 (presubmit successful).