Media Controls: invalidate volume slider when value changes.

Otherwise, the painting will not take into consideration the new
value.

BUG=677903

Review-Url: https://codereview.chromium.org/2616343004
Cr-Commit-Position: refs/heads/master@{#442864}

third_party/WebKit/Source/core/html/shadow/MediaControlElements.cpp

@@ -856,8 +856,12 @@ bool MediaControlVolumeSliderElement::willRespondToMouseClickEvents() {
 }
 
 void MediaControlVolumeSliderElement::setVolume(double volume) {
-  if (value().toDouble() != volume)
+  if (value().toDouble() == volume)
+    return;
+
   setValue(String::number(volume));
+  if (LayoutObject* layoutObject = this->layoutObject())
+    layoutObject->setShouldDoFullPaintInvalidation();
 }
 
 bool MediaControlVolumeSliderElement::keepEventInNode(Event* event) {