Fix assertion failure in range.mm while examining system dictionary popup.

chrome/browser/renderer_host/text_input_client_mac_unittest.mm

Index: chrome/browser/renderer_host/text_input_client_mac_unittest.mm
diff --git a/chrome/browser/renderer_host/text_input_client_mac_unittest.mm b/chrome/browser/renderer_host/text_input_client_mac_unittest.mm
index 6870064e81360133b829b80ae52a0d8db039a655..41551e5c460b86edd8adbae307767c3c3302e6e1 100644
--- a/chrome/browser/renderer_host/text_input_client_mac_unittest.mm
+++ b/chrome/browser/renderer_host/text_input_client_mac_unittest.mm
@@ -106,6 +106,21 @@ TEST_F(TextInputClientMacTest, TimeoutCharacterIndex) {
   EXPECT_EQ(NSNotFound, index);
 }
 
+TEST_F(TextInputClientMacTest, NotFoundCharacterIndex) {
+  ScopedTestingThread thread(this);
+  const NSUInteger kNotFoundValue = static_cast<NSUInteger>(-1);
+
+  PostTask(base::Bind(&TextInputClientMac::SetCharacterIndexAndSignal,
+      base::Unretained(service()), kNotFoundValue));
+  NSUInteger index = service()->GetCharacterIndexAtPoint(
+      widget(), gfx::Point(2, 2));
+
+  EXPECT_EQ(1U, ipc_sink().message_count());
+  EXPECT_TRUE(ipc_sink().GetUniqueMessageMatching(
+      TextInputClientMsg_CharacterIndexForPoint::ID));
+  EXPECT_EQ(NSNotFound, index);
+}
+
 TEST_F(TextInputClientMacTest, GetRectForRange) {
   ScopedTestingThread thread(this);
   const NSRect kSuccessValue = NSMakeRect(42, 43, 44, 45);