Run V8 GC before detecting Document leaks in inspector_memory_unittest.py.

Run V8 GC before detecting Document leaks in inspector_memory_unittest.py.

Previously, this unit test was consistently failing because DOM wrappers were
retained at the time of detecting Document leaks. This patch adds a call to
V8's gc() before the leak check to make sure the DOM wrappers are freed.

BUG=422244
R=kouhei@chromium.org

Committed: https://crrev.com/2c9ef53fe11f7b108b5cd58b5486aae710cdfafe
Cr-Commit-Position: refs/heads/master@{#313471}

[kouhei (in TOK), 2015-01-28 03:09:59]

kouhei@chromium.org changed reviewers:
+ marja@chromium.org

[kouhei (in TOK), 2015-01-28 03:10:00]

+marja: Would you do a OWNER review?

lgtm

[Yuta Kitamura, 2015-01-28 03:54:19]

FYI: The test has passed on Linux.

> [103/762]
telemetry.core.backends.chrome_inspector.inspector_memory_unittest.InspectorMemoryTest.testGetDOMStats
passed

[marja, 2015-01-28 09:46:03]

rubberstamp lgtm

[Yuta Kitamura, 2015-01-28 10:04:31]

The CQ bit was checked by yutak@chromium.org

[commit-bot: I haz the power, 2015-01-28 10:04:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882913002/1

[commit-bot: I haz the power, 2015-01-28 10:09:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-28 10:11:05]

Patchset 1 (id:??) landed as
https://crrev.com/2c9ef53fe11f7b108b5cd58b5486aae710cdfafe
Cr-Commit-Position: refs/heads/master@{#313471}