Android: Speculative fix to a preview tab crash bug

Adds null checks against thinwebview object to attempt to fix the
reported crash bug to do null pointer dereference. This is a
speculative fix since the bug has not been reproducible locally.

Bug: 1030616
Change-Id: I1541133691b33e2d685bd3fd6b0ab08feb3a1e49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1984795Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727939}

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

@@ -56,6 +56,7 @@ public class ThinWebViewImpl extends FrameLayout implements ThinWebView {
 
     @Override
     public void attachWebContents(WebContents webContents, @Nullable View contentView) {
+        if (mNativeThinWebViewImpl == 0) return;
         mWebContents = webContents;
 
         setContentView(contentView);
@@ -66,11 +67,10 @@ public class ThinWebViewImpl extends FrameLayout implements ThinWebView {
 
     @Override
     public void destroy() {
+        if (mNativeThinWebViewImpl == 0) return;
         mCompositorView.destroy();
-        if (mNativeThinWebViewImpl != 0) {
-            ThinWebViewImplJni.get().destroy(mNativeThinWebViewImpl, ThinWebViewImpl.this);
-            mNativeThinWebViewImpl = 0;
-        }
+        ThinWebViewImplJni.get().destroy(mNativeThinWebViewImpl, ThinWebViewImpl.this);
+        mNativeThinWebViewImpl = 0;
     }
 
     @Override
@@ -80,6 +80,7 @@ public class ThinWebViewImpl extends FrameLayout implements ThinWebView {
 
     @Override
     protected void onSizeChanged(int w, int h, int oldw, int oldh) {
+        if (mNativeThinWebViewImpl == 0) return;
         if (w != oldw || h != oldh) {
             ThinWebViewImplJni.get().sizeChanged(
                     mNativeThinWebViewImpl, ThinWebViewImpl.this, w, h);