Fix ConvertJavaStringToUTF8() assertion failure in TabAndroid::GetTitle()

BUG=None

Change-Id: I46a53f84f4017d2b30bdfbe8f100187eaeba6010
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546648Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646602}

chrome/browser/android/tab_android.cc

@@ -159,8 +159,10 @@ int TabAndroid::GetAndroidId() const {
 
 base::string16 TabAndroid::GetTitle() const {
   JNIEnv* env = base::android::AttachCurrentThread();
-  return base::android::ConvertJavaStringToUTF16(
-      Java_Tab_getTitle(env, weak_java_tab_.get(env)));
+  ScopedJavaLocalRef<jstring> java_title =
+      Java_Tab_getTitle(env, weak_java_tab_.get(env));
+  return java_title ? base::android::ConvertJavaStringToUTF16(java_title)
+                    : base::string16();
 }
 
 bool TabAndroid::IsNativePage() const {