[telemetry] Add TraceResult as return value from tracing, and placeholder trace_event_importer.

[telemetry] Add TraceResult as return value from tracing, and placeholder trace_event_importer.

This is infrastructure for adding trace_event parsing to telemetry.

BUG=173327
NOTRY=True

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=179843

[nduca, 2013-01-31 09:29:25]

Tracing previously returned a string formatted as a chrome trace. We try to keep
core telemetry abstracted from what the trace itself is.

telemetry already has a concept of a TimelineModel, which is intended to be a
browser-neutral representation of a perf trace.

Creating this requires importing, so I added an (empty) importer for the
trace_event format that will eventually produce a TimelineModel.

The importing process is not free, and often we just want to write the trace to
file. So, I introduced TraceResult, which wraps the platform-specific result
data and allows it to be dumped right to file in an efficient way, or turned
into a model, only if desired.

Followup patches will implement the importer.

[marja, 2013-01-31 09:41:48]

lgtm with nits:

https://codereview.chromium.org/12082091/diff/1/tools/telemetry/telemetry/tra...
File tools/telemetry/telemetry/tracing_backend.py (right):

https://codereview.chromium.org/12082091/diff/1/tools/telemetry/telemetry/tra...
tools/telemetry/telemetry/tracing_backend.py:25: def WriteToFile(self, f):
As it's not really a file -> "Serialize", "WriteToStream", "WriteToIO" or such
could be a better name.

https://codereview.chromium.org/12082091/diff/1/tools/telemetry/telemetry/tra...
tools/telemetry/telemetry/tracing_backend.py:27: d = self._tracing_data
Can you add here a comment that we're not ",".join()ing because the data is too
big?

[nduca, 2013-01-31 09:47:05]

ty, nits fixed

[commit-bot: I haz the power, 2013-01-31 09:50:22]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nduca@chromium.org/12082091/5001

[marja, 2013-01-31 09:51:06]

https://codereview.chromium.org/12082091/diff/5001/tools/telemetry/telemetry/...
File tools/telemetry/telemetry/trace_event_importer.py (right):

https://codereview.chromium.org/12082091/diff/5001/tools/telemetry/telemetry/...
tools/telemetry/telemetry/trace_event_importer.py:9: def Import(data):
Hmm, we still convert an array of strings to a string, and back, can we avoid
that conversion?

[nduca, 2013-01-31 10:10:59]

Not a bad idea. I'll see what I can do on my followup patch, maybe add a special
case for this. 

One complication is that there are actually several trace event file formats out
in the wild, so if someone tries to open one of those with
trace_event_importer.py, ~shrug~.

The nitty gritty: 
https://code.google.com/p/trace-viewer/source/browse/trunk/src/trace_event_im...

and
https://code.google.com/p/trace-viewer/wiki/TraceEventFormat

[commit-bot: I haz the power, 2013-01-31 10:24:10]

Change committed as 179843