chromeos: makes InputMethodMus notify observers when virtual keyboard shown

Which is used in some tests.

BUG=889101
TEST=covered by tests

Change-Id: I2757361aed21909b8020fe0f9e7c600fe88f950c
Reviewed-on: https://chromium-review.googlesource.com/c/1297408
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602237}

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

 # These tests currently fail with SingleProcessMash enabled.
 # Bug: crbug.com/883523
 
-# Keyboard event related. See crbug.com/889101
--SitePerProcessTextInputManagerTest.CorrectlyShowVirtualKeyboardIfEnabled
-
 # TODO(sky): this now fails because of differences in timing. In particular,
 # in classic mode an accelerator that moves focus is processed *after* text
 # is inserted, where as now the accelerator runs first, resulting in text

ui/aura/mus/input_method_mus.cc

@@ -138,6 +138,7 @@ bool InputMethodMus::IsCandidatePopupOpen() const {
 }
 
 void InputMethodMus::ShowVirtualKeyboardIfEnabled() {
+  InputMethodBase::ShowVirtualKeyboardIfEnabled();
   if (input_method_)
     input_method_->ShowVirtualKeyboardIfEnabled();
 }