Android: Fix assertion failure in RWHVA destruction

Assertion at RenderWidgetHostViewAndroid::StopObservingRootWindow
called via |Destroy| failed, since WebContentsViewAndroid::SetContentViewCore
was passing a valid ViewAndroid instead of nullptr when cvc is destroyed.
This kept RWHVA observing root window, while the view tree was already
torn down by WCVA dtor, leaving RWHVA in inconsistent state where it
couldn't get WindowAndroid any more. This CL fixes the bug. Previously
WCVA passed null CVC to RWHVA before https://crrev.com/c/958646, so
this fix is the correct behavior.

Bug: 828211
Change-Id: I79463cd0dce94c5f8d81417208cd313871c4f5d7
Reviewed-on: https://chromium-review.googlesource.com/991002Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548308}

content/browser/web_contents/web_contents_view_android.cc

@@ -117,7 +117,7 @@ void WebContentsViewAndroid::SetContentViewCore(ContentViewCore* cvc) {
     content_view_core_.reset(cvc);
   RenderWidgetHostViewAndroid* rwhv = GetRenderWidgetHostViewAndroid();
   if (rwhv)
-    rwhv->UpdateNativeViewTree(&view_);
+    rwhv->UpdateNativeViewTree(cvc ? &view_ : nullptr);
 
   if (web_contents_->ShowingInterstitialPage()) {
     rwhv = static_cast<RenderWidgetHostViewAndroid*>(
@@ -127,7 +127,7 @@ void WebContentsViewAndroid::SetContentViewCore(ContentViewCore* cvc) {
             ->GetWidget()
             ->GetView());
     if (rwhv)
-      rwhv->UpdateNativeViewTree(&view_);
+      rwhv->UpdateNativeViewTree(cvc ? &view_ : nullptr);
   }
 }