WebVTT: Support fractional percentage values for position/line/size

Source/core/html/track/vtt/VTTScanner.h

Index: Source/core/html/track/vtt/VTTScanner.h
diff --git a/Source/core/html/track/vtt/VTTScanner.h b/Source/core/html/track/vtt/VTTScanner.h
index 7f193149af6ed131900b98e4ab5f5d7728401955..b036dcac381c567e771c881dabbb493d0b02a01c 100644
--- a/Source/core/html/track/vtt/VTTScanner.h
+++ b/Source/core/html/track/vtt/VTTScanner.h
@@ -129,6 +129,9 @@ public:
     // Scan a floating point value on one of the forms: \d+\.? \d+\.\d+ \.\d+
     bool scanFloat(float& number);
 
+    // Scan a floating point value (per scanFloat) followed by a '%'.
+    bool scanPercentage(float& percentage);
+
 protected:
     Position position() const { return m_data.characters8; }
     Position end() const { return m_end.characters8; }