Add WPT WebAudio tests to blink_audio

Change-Id: Ibfefd9929b5f60b7c0de954ceee75142fdb6a838
Reviewed-on: https://chromium-review.googlesource.com/979016Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545641}

WATCHLISTS

@@ -216,7 +216,8 @@
     },
     'blink_audio': {
       'filepath': 'third_party/WebKit/Source/platform/audio' \
-                  '|third_party/WebKit/Source/modules/webaudio',
+                  '|third_party/WebKit/Source/modules/webaudio' \
+                  '|third_party/WebKit/LayoutTests/external/wpt/webaudio',
     },
     'blink_battery_status': {
       'filepath': 'third_party/WebKit/Source/modules/battery/' \