Avoid double-counting in GpuChannel memory tracking.

Avoid double-counting in GpuChannel memory tracking.

GpuChannel currently calculates current memory usage by iterating over
all stubs and summing their individual memory usage. This results in
double counting as a given stub may share its memory tracker with other
stubs, if they are in a share group.

To avoid double counting, we now iterate over stubs to collect a set of
unique MemoryTrackers, then sum the memory usage of each unique tracker.

BUG=512535
Committed: https://crrev.com/be3028d759f1ef49520f3466b92656af36e7c1de
Cr-Commit-Position: refs/heads/master@{#339746}

[ericrk, 2015-07-21 19:02:46]

ericrk@chromium.org changed reviewers:
+ reveman@chromium.org

[ericrk, 2015-07-21 19:02:54]

ericrk@chromium.org changed reviewers:
- reveman@chromium.org

[ericrk, 2015-07-21 19:04:41]

ericrk@chromium.org changed reviewers:
+ ccameron@chromium.org

[ccameron, 2015-07-21 19:39:54]

Nice -- lgtm.

[ericrk, 2015-07-21 19:52:16]

The CQ bit was checked by ericrk@chromium.org

[commit-bot: I haz the power, 2015-07-21 19:52:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1244843004/1

[commit-bot: I haz the power, 2015-07-21 21:06:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-21 21:07:09]

Patchset 1 (id:??) landed as
https://crrev.com/be3028d759f1ef49520f3466b92656af36e7c1de
Cr-Commit-Position: refs/heads/master@{#339746}