Don't call SpellCheckHost::DisconnectSessionBridge if SpellCheckHost is not bound yet

It's a waste to call SpellCheckHost::DisconnectSessionBridge when
SpellCheckHost is not bound, as it initializes a SpellCheckHost and
then disables it immediately.

Therefore, this patch stops calling it when SpellCheckHost is not
bound yet. This gives a 12% performance improvement on test
blink_perf.css/FocusUpdate:

https://pinpoint-dot-chromeperf.appspot.com/job/15c3ea19740000

Bug: 814225
Change-Id: I0e00a7b31528db424f80cf89ec013751436ea88c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1500779
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637901}

components/spellcheck/renderer/spellcheck_provider.cc

@@ -146,14 +146,14 @@ void SpellCheckProvider::RequestTextChecking(
 
 void SpellCheckProvider::FocusedNodeChanged(const blink::WebNode& unused) {
 #if defined(OS_ANDROID)
+  if (!spell_check_host_.is_bound())
+    return;
+
   WebLocalFrame* frame = render_frame()->GetWebFrame();
   WebElement element = frame->GetDocument().IsNull()
                            ? WebElement()
                            : frame->GetDocument().FocusedElement();
   bool enabled = !element.IsNull() && element.IsEditable();
-
-  // TODO(xiaochengh): Don't call GetSpellCheckHost() if SpellCheckHost is not
-  // bound yet. crbug.com/814225
   if (!enabled)
     GetSpellCheckHost().DisconnectSessionBridge();
 #endif  // defined(OS_ANDROID)