Fix media crash in layout test

Layout test do not use compositor thread, so WebMediaPlayerAndroid ends
up using a null message loop.

BUG=412578

Review URL: https://codereview.chromium.org/586493002

Cr-Commit-Position: refs/heads/master@{#295596}

content/renderer/media/android/webmediaplayer_android.cc

@@ -137,8 +137,11 @@ WebMediaPlayerAndroid::WebMediaPlayerAndroid(
       is_playing_(false),
       needs_establish_peer_(true),
       has_size_info_(false),
+      // Compositor thread does not exist in layout tests.
       compositor_loop_(
-          RenderThreadImpl::current()->compositor_message_loop_proxy()),
+          RenderThreadImpl::current()->compositor_message_loop_proxy()
+              ? RenderThreadImpl::current()->compositor_message_loop_proxy()
+              : base::MessageLoopProxy::current()),
       stream_texture_factory_(factory),
       needs_external_surface_(false),
       video_frame_provider_client_(NULL),