[VK] Log lingering states.

We currently have several tast integration tests that test the VK under
real devices. However, in rare cases, the VK doesn't show up and it's
not clear why.

One route of investigaiton is to first determine whether the issue is
due to C++ or JavaScript. We put an error log when there's a lingering
state. If we linger in LOADING_EXTENSION, that probably means it's a
C++ issue, as the extension should load in under 5s (if the extension
has a runtime error, it also counts as "loaded"). If we don't linger,
then it means the extension is not resizing the keyboard window.

Bug: 879073
Change-Id: I60945fd1f5c2dd8284c0ea416b4127e12a9d394c
Reviewed-on: https://chromium-review.googlesource.com/c/1436277
Auto-Submit: Darren Shen <shend@chromium.org>
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626469}

ui/keyboard/keyboard_controller.cc

@@ -990,6 +990,7 @@ void KeyboardController::ChangeState(KeyboardControllerState state) {
 }
 
 void KeyboardController::ReportLingeringState() {
+  LOG(ERROR) << "KeyboardController lingering in " << StateToStr(state_);
   UMA_HISTOGRAM_ENUMERATION("VirtualKeyboard.LingeringIntermediateState",
                             state_, KeyboardControllerState::COUNT);
 }