TaskManager: Remove some unused/unnecessary methods.

TaskManager: Remove some unused/unnecessary methods.

What's removed:
 - The idea of unique_id's (these were for the now-gone webUI taskmanager).
 - The GetResourceExtension() method, which the tests no longer need.
 - The IsBackground getter, which is not used.

BUG=348836
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255217

[ncarter (slow), 2014-03-03 22:05:13]

Hi Avi,

Please have a look (this is the second in the series, and depends on the browser
test rewrite). This one is just cleanup.

[Avi (use Gerrit), 2014-03-03 22:55:03]

This looks good, but requires a bug #. Add in the same tracking bug # that you
use for http://crrev.com/185873003.

[ncarter (slow), 2014-03-04 01:54:42]

yoshiki@chromium.org: Please review everything (as task_manager OWNER)

[Avi (use Gerrit), 2014-03-04 02:11:59]

FWIW it LGTM.

[yoshiki, 2014-03-05 18:34:13]

task manager lgtm. Thanks for doing this.

[ncarter (slow), 2014-03-05 19:46:09]

The CQ bit was checked by nick@chromium.org

[commit-bot: I haz the power, 2014-03-05 19:48:01]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nick@chromium.org/186153002/1

[commit-bot: I haz the power, 2014-03-05 20:21:57]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-05 20:21:57]

Retried try job too often on win for step(s) compile
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win&number...

[ncarter (slow), 2014-03-05 20:25:21]

The CQ bit was checked by nick@chromium.org

[commit-bot: I haz the power, 2014-03-05 20:26:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nick@chromium.org/186153002/1

[commit-bot: I haz the power, 2014-03-05 22:40:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-05 22:40:35]

Retried try job too often on win_rel for step(s) browser_tests, net_unittests,
unit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[ncarter (slow), 2014-03-05 22:46:53]

The CQ bit was checked by nick@chromium.org

[commit-bot: I haz the power, 2014-03-05 22:48:45]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nick@chromium.org/186153002/1

[commit-bot: I haz the power, 2014-03-06 00:08:13]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nick@chromium.org/186153002/1

[commit-bot: I haz the power, 2014-03-06 01:59:05]

Change committed as 255217