Use estimated vsync period on Android

ui/android/java/src/org/chromium/ui/VSyncMonitor.java

Index: ui/android/java/src/org/chromium/ui/VSyncMonitor.java
diff --git a/ui/android/java/src/org/chromium/ui/VSyncMonitor.java b/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
index af8f87b5ffd898c832a1fa2f0db7cd590e50ca89..13f9d01e6bf78f2a4d26a51dfb337f8f56f47905 100644
--- a/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
+++ b/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
@@ -41,7 +41,7 @@ public class VSyncMonitor {
     private Listener mListener;
 
     // Display refresh rate as reported by the system.
-    private final long mRefreshPeriodNano;
+    private long mRefreshPeriodNano;
 
     private boolean mHaveRequestInFlight;
 
@@ -89,6 +89,14 @@ public class VSyncMonitor {
                 @Override
                 public void doFrame(long frameTimeNanos) {
                     TraceEvent.begin("VSync");
+                    // Display.getRefreshRate() is unreliable on some platforms.

[Sami, 2014/09/30 16:04:14]

Can we make this logic conditional on whether getRefreshRate() looks bogus or
not? I'd rather trust the system if it can give us a correct interval.

[jmanko, 2014/10/01 09:36:51]

Acknowledged.

+                    // Adjust refresh period- initial value is based on Display.getRefreshRate(),
+                    // after that it asymptotically approaches the real value.
+                    // Last refresh duration is capped at 30 fps
+                    long lastRefreshDurationNano =
+                        Math.min(frameTimeNanos - mGoodStartingPointNano,
+                                 34 * NANOSECONDS_PER_MILLISECOND);
+                    mRefreshPeriodNano = (mRefreshPeriodNano + lastRefreshDurationNano) / 2;

[Sami, 2014/09/30 16:04:14]

Instead of capping arbitrarily, can we just update the estimate only if we know
that we saw two consecutive vsyncs?

[jmanko, 2014/10/01 09:36:51]

You mean something like this?
long lastRefreshDurationNano = frameTimeNanos - mGoodStartingPointNano;
if (lastRefreshDurationNano < 34 * NANOSECONDS_PER_MILLISECOND)
    mRefreshPeriodNano = (mRefreshPeriodNano + lastRefreshDurationNano) / 2;

[Sami, 2014/10/01 09:39:34]

That would work. Please also make the |34 * NANOSECONDS_PER_MILLISECOND| a named
constant.

[no sievers, 2014/10/01 17:48:09]

Can't this be done easier and in a more robust way because we know whether we
requested a follow-up vsync tick after a given vsync tick?
I feel like we should rather not use mGoodStartingPointNano if it wasn't set in
the previous vsync.

                     mGoodStartingPointNano = frameTimeNanos;
                     onVSyncCallback(frameTimeNanos, getCurrentNanoTime());
                     TraceEvent.end("VSync");
@@ -132,7 +140,7 @@ public class VSyncMonitor {
     /**
      * Determine whether a true vsync signal is available on this platform.
      */
-    private boolean isVSyncSignalAvailable() {
+    public boolean isVSyncSignalAvailable() {
         return mChoreographer != null;
     }