[Presentation API] Implement ondefaultsessionstart in PSImpl.

[Presentation API] Implement ondefaultsessionstart in PSImpl.

Added DefaultSessionStartContext for coordinating sending default
session back to PresentationDispatcher.

When ListenForDefaultSessionStart is called, DefaultSessionStartContext
will be installed on PresentationServiceImpl. When both the default
session and PresentationDispatcher's callback are available, the
callback will be invoked with the session.

On Reset(), if a callback is available, it will be invoked with null.
Changed PresentationDispatcher to not update Blink in that case.

Also, PSImpl now keeps track of the corresponding RFH's ID instead of
RFH* since most of the time we only need to use the ID.

Changed PresentationServiceDelegate's Add/RemoveObserver interface,
since the PresentationServiceDelegate need to be able to correlate
an Observer with a RFH. (at most 1 per RFH, as it stands today).

Added OnDefaultPresentationStarted to
PresentationServiceDelegate::Observer interface and implemented it
in PresentationServiceImpl.

Added tests in PresentationServiceImpl.

BUG=459001
Committed: https://crrev.com/79ddfb2383dca537e481eb9e0a477267dd48bc60
Cr-Commit-Position: refs/heads/master@{#326353}

[imcheng, 2015-04-15 20:39:20]

Patchset #1 (id:1) has been deleted

[imcheng, 2015-04-17 02:11:30]

imcheng@chromium.org changed reviewers:
+ avayvod@google.com, avi@chromium.org, imcheng@chromium.org,
mfoltz@chromium.org

[imcheng, 2015-04-17 02:11:31]

Avi: PTAL at content/public/browser/presentation_service_delegate.h
Anton/mark: PTAL at everything.

Thanks.

[Avi (use Gerrit), 2015-04-17 02:58:04]

content/public/browser/presentation_service_delegate.h lgtm

[imcheng, 2015-04-20 21:30:02]

Ping for mark / Anton.

[whywhat, 2015-04-20 22:16:35]

avayvod@chromium.org changed reviewers:
+ avayvod@chromium.org

[whywhat, 2015-04-20 22:16:36]

lgtm

[mark a. foltz, 2015-04-21 21:39:24]

lgtm

[imcheng, 2015-04-21 23:49:25]

Patchset #2 (id:40001) has been deleted

[imcheng, 2015-04-21 23:49:38]

Patchset #3 (id:80001) has been deleted

[imcheng, 2015-04-22 18:48:16]

The CQ bit was checked by imcheng@chromium.org

[imcheng, 2015-04-22 18:48:16]

The patchset sent to the CQ was uploaded after l-g-t-m from mfoltz@chromium.org,
avi@chromium.org, avayvod@chromium.org
Link to the patchset: https://codereview.chromium.org/1073893003/#ps180001
(title: "use RunLoopFor")

[commit-bot: I haz the power, 2015-04-22 18:50:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1073893003/180001

[commit-bot: I haz the power, 2015-04-22 19:07:26]

Committed patchset #7 (id:180001)

[commit-bot: I haz the power, 2015-04-22 19:09:25]

Patchset 7 (id:??) landed as
https://crrev.com/79ddfb2383dca537e481eb9e0a477267dd48bc60
Cr-Commit-Position: refs/heads/master@{#326353}

[vmpstr, 2015-04-22 22:08:36]

A revert of this CL (patchset #7 id:180001) has been created in
https://codereview.chromium.org/1102683002/ by vmpstr@chromium.org.

The reason for reverting is: Speculative revert to fix content_unittests
failures:
http://build.chromium.org/p/chromium.linux/builders/Android%20Tests%20%28dbg%....