[CastRunner] Allow Agent's enable-touch bindings to supercede the Runner's.

Prevents the CastRunner from clobbering the Agent's
setTouchSupportEnabled() platform API if it's already been installed.

Bug: 953796,953958
Change-Id: Ia1044a96209f04d792a8c671a8ac18eb98b2d9ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728445Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683227}

fuchsia/runners/cast/touch_input_bindings.js

@@ -4,7 +4,9 @@
 
 'use strict';
 
-cast.__platform__.__touchInput__ = new class {
+// Don't supercede an Agent-provided API function.
+if (!cast.__platform__.setTouchInputSupport) {
+  cast.__platform__.__touchInput__ = new class {
     constructor() {
       this.port_ = cast.__platform__.PortConnector.bind(
           'cast.__platform__.__touchInput__');
@@ -60,14 +62,15 @@ cast.__platform__.__touchInput__ = new class {
     // bindings layer.
     port_ = null;
 
-  // A dictionary relating inflight request IDs to their Promise resolve/reject
-  // functions.
+    // A dictionary relating inflight request IDs to their Promise
+    // resolve/reject functions.
     pendingRequests_ = {};
 
     // Unique ID of the next request.
     currentRequestId_ = 0;
-};
+  };
 
-cast.__platform__.setTouchInputSupport =
+  cast.__platform__.setTouchInputSupport =
       cast.__platform__.__touchInput__.setTouchInputSupport.bind(
           cast.__platform__.__touchInput__);
+}