Mojo: Make mojo::Handle::value() return a reference instead of a value.

Mojo: Make mojo::Handle::value() return a reference instead of a value.

Getting the value is fine and fast, but returning a reference has the
advantage of allowing you to pass &handle.value() to anything taking a
|const MojoHandle*| (without needing to save to a temporary variable).

R=sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285019

[sky, 2014-07-23 16:53:20]

LGTM

[viettrungluu, 2014-07-23 19:51:30]

Committed patchset #1 manually as r285019 (presubmit successful).