Remove NativeViewportClient

Remove NativeViewportClient

This removes the client interface from NativeViewport. This interface
had two methods, OnDestroyed and OnMetricsChanged. OnDestroyed is
redundant with just closing the message pipe. OnMetricsChanged is more
interesting. Since there's no rate limiting in this interface, an
unbounded number of metrics updates could accumulate in this pipe. Since
users of the NativeViewport service will (generally) only care about
the most recent update this replaces the pipe with a request->callback
pair NativeViewport.RequestMetrics. The reply message to this is sent
whenever the metrics change. Callers have been updated to either use
the metrics from the initial creation callback (for samples that do not
handle resize) or continually call RequestMetrics if they wish to track
size / scale changes. This way only one request is pending per
connection.

BUG=451319
R=sky@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/91b812b1af824c8faba2f75d88d31a1532164728

[jamesr, 2015-01-27 23:48:54]

jamesr@chromium.org changed reviewers:
+ darin@chromium.org

[jamesr, 2015-01-28 00:45:26]

jamesr@chromium.org changed reviewers:
+ sky@chromium.org

[jamesr, 2015-01-28 00:52:33]

FYI Eric - this touches some files that
https://codereview.chromium.org/880743002/ touches.  I don't think it really
conflicts (it's mostly changing a different layer), but if you're close to
landing that one I'm happy to rebase this if you want to land that first.

[sky, 2015-01-28 19:17:33]

https://codereview.chromium.org/878933005/diff/20001/mojo/services/native_vie...
File mojo/services/native_viewport/public/interfaces/native_viewport.mojom
(right):

https://codereview.chromium.org/878933005/diff/20001/mojo/services/native_vie...
mojo/services/native_viewport/public/interfaces/native_viewport.mojom:22:
RequestMetrics() => (ViewportMetrics metrics);
With this model how would someone know when the metrics change?

[sky, 2015-01-28 19:26:41]

James clarified how this works in person. LGTM

https://codereview.chromium.org/878933005/diff/20001/examples/sample_app/samp...
File examples/sample_app/sample_app.cc (right):

https://codereview.chromium.org/878933005/diff/20001/examples/sample_app/samp...
examples/sample_app/sample_app.cc:58: base::Bind(&SampleApp::OnMetricsChanged,
weak_factory_.GetWeakPtr()));
As this class owns the viewport_ it seems like it doesn't need weak factory, but
that can be dealt with separately.

[jamesr, 2015-01-28 21:32:58]

Added a comment and removed the unnecessary WeakPtrFactory from sample_app.

[jamesr, 2015-01-29 01:13:59]

Committed patchset #2 (id:20001) manually as
91b812b1af824c8faba2f75d88d31a1532164728 (presubmit successful).