Add mojo service to get notified of vsync.

mojo/services/vsync/interfaces/vsync.mojom

Index: mojo/services/vsync/interfaces/vsync.mojom
diff --git a/mojo/services/icu_data/interfaces/icu_data.mojom b/mojo/services/vsync/interfaces/vsync.mojom
similarity index 53%
copy from mojo/services/icu_data/interfaces/icu_data.mojom
copy to mojo/services/vsync/interfaces/vsync.mojom
index ced5aa64359941c8bcdc962d2ace971239b1e5f2..e4d7dda296bfabc5f41517afa3a888c111111b0b 100644
--- a/mojo/services/icu_data/interfaces/icu_data.mojom
+++ b/mojo/services/vsync/interfaces/vsync.mojom
@@ -3,8 +3,10 @@
 // found in the LICENSE file.
 
 [DartPackage="mojo_services"]
-module icu_data;
+module vsync;
 
-interface ICUData {
-  Map(string sha1hash) => (handle<shared_buffer>? icu_data);
+interface VSyncProvider {
+  // Timebase is in MojoGetTimeTicksNow.

[ppi, 2015/11/17 16:40:46]

Please document what this does.

E.g. Waits for the next vsync and returns its timestamp once it happens.

[ppi, 2015/11/17 16:40:46]

The source of this timestamp is actually not MojoGetTimeTicksNow(), we get this
from the OS and it represents time "since boot, not counting time spent in deep
sleep.". Is this in line with what we use for mojo timeticks on Android?

[qsr, 2015/11/17 17:19:40]

Done.

[qsr, 2015/11/17 17:19:40]

Removed any reference of the timestamp, saying that it can only be compared with
other calls of this method.

+  // Only one callback can be parked at a given time.
+  AwaitVSync() => (int64 time_stamp);
 };