Fail gracefully if the postMessage channel is closed

chrome/android/java/src/org/chromium/chrome/browser/customtabs/PostMessageHandler.java

Index: chrome/android/java/src/org/chromium/chrome/browser/customtabs/PostMessageHandler.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/PostMessageHandler.java b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/PostMessageHandler.java
index cc5ebccc319cfc7c40ed160b60a63db5d170e6f9..268a1a3c2acc6b7f1bf1751f2beec72b68901962 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/PostMessageHandler.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/PostMessageHandler.java
@@ -143,6 +143,9 @@ public class PostMessageHandler {
         ThreadUtils.postOnUiThread(new Runnable() {
             @Override
             public void run() {
+                // It is still possible that the page has navigated while this task is in the queue.
+                // If that happens fail gracefully.
+                if (mChannel == null || mChannel[0].isClosed()) return;

[jbudorick, 2016/11/15 19:13:12]

Should we do something to indicate this kind of failure?

I guess at this point, we've already returned RESULT_SUCCESS, so perhaps there's
nothing sensible to do here.

[Yusuf, 2016/11/15 19:16:32]

Yeah, the return is more about accepting the request than an actual message
being delivered, since it can actually fail across the pipeline in different
ways as well. postMessage protocol doesn't really make guarantees around that.
At this point there is not much we can do other than not crash.

                 mChannel[0].postMessage(message, null);
             }
         });