content/renderer/media: Use collectAllGarbageForTesting in tests

Use WebHeap::collectAllGarbageForTesting instead of
WebHeap::collectGarbageForTesting.

Background:
Blink memory team is shipping a mark-and-sweep GC (Oilpan) to
media stream classes, and those classes have many references
between Chromium and Blink.
If a reference chain has multiple references between Chromium
and Blink, collectGarbageForTesting collects one of them.
Instead, collectAllGarageForTesting is expected to collect
all references.

BUG=None

Committed: https://crrev.com/ed516c60b6cac51ef3d7a1e39c9e063daf14d787
Cr-Commit-Position: refs/heads/master@{#352471}

[peria, 2015-10-05 02:32:28]

peria@chromium.org changed reviewers:
+ haraken@chromium.org, mcasas@chromium.org

[peria, 2015-10-05 02:32:28]

PTL

[haraken, 2015-10-05 02:32:53]

LGTM

[mcasas, 2015-10-05 15:54:42]

lgtm

[haraken, 2015-10-05 23:25:39]

The CQ bit was checked by haraken@chromium.org

[commit-bot: I haz the power, 2015-10-05 23:26:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1383803003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1383803003/1

[commit-bot: I haz the power, 2015-10-06 00:05:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-06 00:06:32]

Patchset 1 (id:??) landed as
https://crrev.com/ed516c60b6cac51ef3d7a1e39c9e063daf14d787
Cr-Commit-Position: refs/heads/master@{#352471}