[ThinWebView] Make sure ContentView is detached upon TWV destruction

ContentView was not being removed together with WebContents wrapped
in ThinWebView. This CL makes sure to remove it from the view
hierarchy.

Bug: 1080552
Change-Id: I54c81d8c0b67b35340af0a875c671d4bdd119599
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212240Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772557}

chrome/browser/android/thin_webview/internal/java/src/org/chromium/chrome/browser/thinwebview/internal/ThinWebViewImpl.java

@@ -77,6 +77,10 @@ public class ThinWebViewImpl extends FrameLayout implements ThinWebView {
     @Override
     public void destroy() {
         if (mNativeThinWebViewImpl == 0) return;
+        if (mContentView != null) {
+            removeView(mContentView);
+            mContentView = null;
+        }
         mCompositorView.destroy();
         ThinWebViewImplJni.get().destroy(mNativeThinWebViewImpl, ThinWebViewImpl.this);
         mNativeThinWebViewImpl = 0;