[Android] Avoid unnecessary JNI calls for empty string conversions

base/android/jni_string.cc

Index: base/android/jni_string.cc
diff --git a/base/android/jni_string.cc b/base/android/jni_string.cc
index 57c2805ead39177d462daba5aa345bfd9026973d..121755ab75e46cfe6017cdae28f3870056498525 100644
--- a/base/android/jni_string.cc
+++ b/base/android/jni_string.cc
@@ -29,12 +29,18 @@ void ConvertJavaStringToUTF8(JNIEnv* env, jstring str, std::string* result) {
     result->clear();
     return;
   }
+  const jsize length = env->GetStringLength(str);
+  if (!length) {
+    result->clear();
+    CheckException(env);
+    return;
+  }
   // JNI's GetStringUTFChars() returns strings in Java "modified" UTF8, so
   // instead get the String in UTF16 and convert using chromium's conversion
   // function that yields plain (non Java-modified) UTF8.
   const jchar* chars = env->GetStringChars(str, NULL);
   DCHECK(chars);
-  UTF16ToUTF8(chars, env->GetStringLength(str), result);
+  UTF16ToUTF8(chars, length, result);
   env->ReleaseStringChars(str, chars);
   CheckException(env);
 }
@@ -69,11 +75,17 @@ void ConvertJavaStringToUTF16(JNIEnv* env, jstring str, string16* result) {
     result->clear();
     return;
   }
+  const jsize length = env->GetStringLength(str);
+  if (!length) {
+    result->clear();
+    CheckException(env);
+    return;
+  }
   const jchar* chars = env->GetStringChars(str, NULL);
   DCHECK(chars);
   // GetStringChars isn't required to NULL-terminate the strings
   // it returns, so the length must be explicitly checked.
-  result->assign(chars, env->GetStringLength(str));
+  result->assign(chars, length);
   env->ReleaseStringChars(str, chars);
   CheckException(env);
 }