Add Blink side support for SourceBuffer.appendWindowStart & SourceBuffer.appendWindowEnd.

public/web/WebSourceBuffer.h

Index: public/web/WebSourceBuffer.h
diff --git a/public/web/WebSourceBuffer.h b/public/web/WebSourceBuffer.h
index 07c433a27893300918faa13927bce6807fa20227..f2cd1954bf9f352080b88fcda59e8d35474915c1 100644
--- a/public/web/WebSourceBuffer.h
+++ b/public/web/WebSourceBuffer.h
@@ -44,6 +44,13 @@ public:
     virtual void abort() = 0;
     virtual void remove(double start, double end) = 0;
     virtual bool setTimestampOffset(double) = 0;
+
+    // Set presentation timestamp for the start of append window.
+    virtual void setAppendWindowStart(double) = 0;
+
+    // Set presentation timestamp for the end of append window.
+    virtual void setAppendWindowEnd(double) = 0;
+
     virtual void removedFromMediaSource() = 0;
 };