android: remark a pure virtual method as such.

A previous commit (791fb7fd)
inadvertently
reverted 8564fcee that's required to fix
a downstream bot. Relanding it.

From the initial commit:

    This causes link issues with a "missing key method" for the orderfile
    bot. This is due to a virtual member function not marked as pure, yet
    not implemented.

Bug: 767836
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ifa8b02eaf19c7475882823499228974e2d75cd55
TBR: watk
Reviewed-on: https://chromium-review.googlesource.com/702395Reviewed-by: Matthew Cary <mattcary@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506754}

media/gpu/android/android_video_surface_chooser.h

@@ -62,8 +62,9 @@ class MEDIA_GPU_EXPORT AndroidVideoSurfaceChooser {
 
   // Sets the client callbacks to be called when a new surface choice is made.
   // Must be called before UpdateState();
-  virtual void SetClientCallbacks(UseOverlayCB use_overlay_cb,
-                                  UseSurfaceTextureCB use_surface_texture_cb);
+  virtual void SetClientCallbacks(
+      UseOverlayCB use_overlay_cb,
+      UseSurfaceTextureCB use_surface_texture_cb) = 0;
 
   // Updates the current state and makes a new surface choice with the new
   // state. If |new_factory| is empty, the factory is left as-is. Otherwise,