Change FilterChain::Append() to a template method.

Change FilterChain::Append() to a template method.

So that we can avoid creating PassThroughFilter.
The CL also makes FilterChain::GetHead() a little more efficient (SyncDispatcher calls it once for receiving each message).

BUG=None
TEST=None
R=darin@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=271107

[yzshen1, 2014-05-16 09:05:22]

Hi, Darin.

Would you please take a look? Thanks!

[darin (slow to review), 2014-05-16 18:19:06]

LGTM

[yzshen1, 2014-05-16 18:45:15]

The CQ bit was checked by yzshen@chromium.org

[commit-bot: I haz the power, 2014-05-16 18:46:23]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yzshen@chromium.org/291583005/1

[commit-bot: I haz the power, 2014-05-16 19:20:01]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)

[commit-bot: I haz the power, 2014-05-16 19:44:33]

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

[commit-bot: I haz the power, 2014-05-16 19:44:33]

Try jobs failed on following builders:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)

[yzshen1, 2014-05-16 21:53:10]

Committed patchset #1 manually as r271107 (presubmit successful).