weblayer: add assertions in Browser creation

Adds two assertion to BrowserFragmentImpl.OnCreate.

1. that mBrowser is null, otherwise we're creating a new one. I believe this
   is valid as onCreate() is only called once.
2. that mContext is not-null. My understanding is onCreate() is always called
   after onAttach(), so that mContext should not be null.

BUG=none
TEST=covered by tests

Change-Id: Ic863a8a63406357a867453690be706e55039cb23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2030049Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736901}

weblayer/browser/java/org/chromium/weblayer_private/BrowserFragmentImpl.java

@@ -52,10 +52,12 @@ public class BrowserFragmentImpl extends RemoteFragmentImpl {
     public void onCreate(Bundle savedInstanceState) {
         StrictModeWorkaround.apply();
         super.onCreate(savedInstanceState);
+        // onCreate() is only called once
+        assert mBrowser == null;
         mBrowser = new BrowserImpl(mProfile, mPersistenceId, savedInstanceState);
-        if (mContext != null) {
-            mBrowser.onFragmentAttached(mContext, new FragmentWindowAndroid(mContext, this));
-        }
+        // onCreate() is always called after onAttach(). onAttach() sets |mContext|.
+        assert mContext != null;
+        mBrowser.onFragmentAttached(mContext, new FragmentWindowAndroid(mContext, this));
     }
 
     @Override