Android: Add 'assert !mAlwaysInForeground' for dropOomBindings()

Follow-up on comment from https://codereview.chromium.org/889013002/
TBR=ppi@chromium.org

Review URL: https://codereview.chromium.org/902593003

Cr-Commit-Position: refs/heads/master@{#314659}

content/public/android/java/src/org/chromium/content/browser/ChildProcessConnectionImpl.java

@@ -415,6 +415,7 @@ public class ChildProcessConnectionImpl implements ChildProcessConnection {
     @Override
     public void dropOomBindings() {
         synchronized (mLock) {
+            assert !mAlwaysInForeground;
             mInitialBinding.unbind();
 
             mStrongBindingCount = 0;