chromeos: makes InputMethodMus notify observers when caret bounds change

Which is used in some tests.

BUG=889101
TEST=covered by tests

Change-Id: I74e191dd1bb7576d382e57b10f119bb4f6d4457b
Reviewed-on: https://chromium-review.googlesource.com/c/1297404Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602171}

testing/buildbot/filters/chromeos.single_process_mash.interactive_ui_tests.filter

@@ -3,9 +3,6 @@
 
 # Keyboard event related. See crbug.com/889101
 -SitePerProcessTextInputManagerTest.CorrectlyShowVirtualKeyboardIfEnabled
--TextInput_SurroundingTextChangedTest.FocusToTextContainingTextAreaByClickingCase
--TextInput_SurroundingTextChangedTest.SurroundingTextChangedWithComposition
--TextInput_SurroundingTextChangedTest.SurroundingTextChangedWithInsertText
 
 # TODO(sky): this now fails because of differences in timing. In particular,
 # in classic mode an accelerator that moves focus is processed *after* text

ui/aura/mus/input_method_mus.cc

@@ -112,6 +112,8 @@ void InputMethodMus::OnCaretBoundsChanged(const ui::TextInputClient* client) {
 
   if (input_method_)
     input_method_->OnCaretBoundsChanged(client->GetCaretBounds());
+
+  NotifyTextInputCaretBoundsChanged(client);
 }
 
 void InputMethodMus::CancelComposition(const ui::TextInputClient* client) {