Enable round-tripping and updating of SourceBuffer timestamp offset

public/platform/WebSourceBuffer.h

Index: public/platform/WebSourceBuffer.h
diff --git a/public/platform/WebSourceBuffer.h b/public/platform/WebSourceBuffer.h
index e789648be8f95d62073d2a76d7bdaf74fbe6a0c4..0349166e85731ce391f2947fd90b711b1c4775b1 100644
--- a/public/platform/WebSourceBuffer.h
+++ b/public/platform/WebSourceBuffer.h
@@ -50,6 +50,13 @@ public:
     virtual void remove(double start, double end) = 0;
     virtual bool setTimestampOffset(double) = 0;
 
+    // Retrieve the implementation's updated timestamp offset, if any. If the
+    // implementation has not updated the offset since its construction or the
+    // last setTimestampOffset(), returns NaN instead of the actual timestamp
+    // offset. This allows caller to exactly round-trip a user-specified offset
+    // and implementation to update the offset.
+    virtual double updatedTimestampOffset() = 0;

[acolwell GONE FROM CHROMIUM, 2014/02/25 21:53:53]

nit: Add a default implementation that returns NaN so this can be landed
independent of the Chromium side change.

[wolenetz, 2014/02/25 22:34:35]

Neat! Done.

+
     // Set presentation timestamp for the start of append window.
     virtual void setAppendWindowStart(double) = 0;