PrecacheFetcher::CancelPrecaching should run in the main UI thread

chrome/android/java/src/org/chromium/chrome/browser/precache/PrecacheController.java

Index: chrome/android/java/src/org/chromium/chrome/browser/precache/PrecacheController.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/precache/PrecacheController.java b/chrome/android/java/src/org/chromium/chrome/browser/precache/PrecacheController.java
index 4ee943e13f58bed495e048134c90bffab5038e92..6b48cd6a41114c010f76f053dcd075e9e2509165 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/precache/PrecacheController.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/precache/PrecacheController.java
@@ -21,6 +21,7 @@ import com.google.android.gms.gcm.PeriodicTask;
 
 import org.chromium.base.ContextUtils;
 import org.chromium.base.Log;
+import org.chromium.base.ThreadUtils;
 import org.chromium.base.VisibleForTesting;
 import org.chromium.base.library_loader.LibraryLoader;
 import org.chromium.base.metrics.RecordHistogram;
@@ -413,13 +414,18 @@ public class PrecacheController {
      * Cancels the current precache session.
      * @param event the failure reason.
      */
-    private void cancelPrecaching(int event) {
-        Log.v(TAG, "canceling precache session");
-        if (setIsPrecaching(false)) {
-            mPrecacheLauncher.cancel();
-            shutdownPrecaching(true);
-        }
-        PrecacheUMA.record(event);
+    private void cancelPrecaching(final int event) {

[Raj, 2016/07/08 17:57:35]

This could be called from PrecacheManager::Shutdown() or other places where it
could be a different thread.

[sclittle, 2016/07/11 18:16:51]

Interesting, could you put this comment in the code? If it's worth mentioning
here then it seems like it would probably be useful to future readers of this
code.

[Raj, 2016/07/12 00:32:26]

Done.

+        ThreadUtils.runOnUiThread(new Runnable() {
+            @Override
+            public void run() {
+                Log.v(TAG, "canceling precache session");
+                if (setIsPrecaching(false)) {
+                    mPrecacheLauncher.cancel();
+                    shutdownPrecaching(true);
+                }
+                PrecacheUMA.record(event);
+            }
+        });
     }
 
     /**