Reland of "[tracing] Add browser-side tests for memory dumps."

Reland of "[tracing] Add browser-side tests for memory dumps."

This is identical to https://crrev.com/1290103004. Once
https://crrev.com/1289793007 lands the tests should no longer be flaky.

Original description below:

The original CL (https://crrev.com/1295753002) got reverted due to test
flakiness. This was because of the memory dump manager performing a
periodic memory dump. If the test completed before such a dump would be
done, it would pass, but when a periodic dump happened during the test,
OnMemoryDump would be called too often. The solution is to disable
periodic memory dumps in tests. This needs to be done via TraceConfig in
the long term, for now this is a workaround.

BUG=518823, 522009, 522165
TBR=dsinclair@chromium.org

Committed: https://crrev.com/6cbb92a6de7198fbf824e8b50c10ae115d789e1a
Cr-Commit-Position: refs/heads/master@{#345076}

[Ruud van Asseldonk, 2015-08-24 12:00:13]

ruuda@google.com changed reviewers:
+ primiano@google.com

[Ruud van Asseldonk, 2015-08-24 12:00:14]

Once https://crrev.com/1289793007 lands we can land this again and the flakiness
should be fixed. I have an other CL in the queue which disables registration of
all non-testing memory dump providers during tests, to ensure that the tests
will not go flaky in the future due to an unrelated change.

[Primiano Tucci (use gerrit), 2015-08-24 14:42:09]

primiano@chromium.org changed reviewers:
+ primiano@chromium.org

[Primiano Tucci (use gerrit), 2015-08-24 14:42:09]

LGTM. Please keep a close eye to the build.chromium.org waterfall and
sheriff-o-matic.appspot.com to see if we are re-introducing some flakiness.

CC +thakis: we'll keep an eye on the waterfalls. FYI in case we miss it or
becomes flaky again in MTV timezone.

[Ruud van Asseldonk, 2015-08-24 14:55:01]

The CQ bit was checked by ruuda@google.com

[commit-bot: I haz the power, 2015-08-24 14:55:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1308443002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1308443002/20001

[commit-bot: I haz the power, 2015-08-24 15:49:05]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-24 15:49:42]

Patchset 2 (id:??) landed as
https://crrev.com/6cbb92a6de7198fbf824e8b50c10ae115d789e1a
Cr-Commit-Position: refs/heads/master@{#345076}