ConvolverOptions.buffer should be nullable

TBR=rtoy@chromium.org,foolip@chromium.org
BUG=626449,647693

Review-Url: https://codereview.chromium.org/2352463002
Cr-Commit-Position: refs/heads/master@{#419388}

third_party/WebKit/Source/modules/webaudio/ConvolverOptions.idl

@@ -4,6 +4,6 @@
 
 // See https://webaudio.github.io/web-audio-api/#convolveroptions
 dictionary ConvolverOptions : AudioNodeOptions {
-    AudioBuffer buffer;
+    AudioBuffer? buffer;
     boolean disableNormalization = false;
-};
\ No newline at end of file
+};