[Android WebView] Implement a better OnReceivedError callback

android_webview/java/src/org/chromium/android_webview/AwContentsStatics.java

Index: android_webview/java/src/org/chromium/android_webview/AwContentsStatics.java
diff --git a/android_webview/java/src/org/chromium/android_webview/AwContentsStatics.java b/android_webview/java/src/org/chromium/android_webview/AwContentsStatics.java
index dab4274cd8f1de439061e5963724c8e255a633fd..f38c73425150ff99cad344a90015a25ae2b115d0 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwContentsStatics.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwContentsStatics.java
@@ -64,6 +64,10 @@ public class AwContentsStatics {
     }
 
     public static String getUnreachableWebDataUrl() {
+        // Note that this method may be called from both IO and UI threads,
+        // but as it only retrieves a value of a constant from native, even if
+        // two calls will be running at the same time, this should not cause
+        // any harm.
         if (sUnreachableWebDataUrl == null) {
             sUnreachableWebDataUrl = nativeGetUnreachableWebDataUrl();
         }