VR: Fix wrong Activity being resumed sometimes when cancelling animation

Android task stacks are a confusing mess. For some reason starting the
animation cancel activity in its own task stack is leading to a less-
recent CTA instance being resumed instead of the CCT that invoked the
animation cancel.

We can fix this by not starting the animation cancel in a new task
stack.

Bug: 817476, 817503
Change-Id: I99fe4583c02c9c5c2333697166e6e4081ad4f818
Reviewed-on: https://chromium-review.googlesource.com/941912Reviewed-by: Yash Malik <ymalik@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539984}

chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrShellDelegate.java

@@ -1286,9 +1286,12 @@ public class VrShellDelegate
         if (DEBUG_LOGS) Log.e(TAG, "canceling startup animation");
         mCancellingEntryAnimation = true;
         Bundle options = ActivityOptions.makeCustomAnimation(mActivity, 0, 0).toBundle();
-        mActivity.startActivity(mVrDaydreamApi.setupVrIntent(
-                                        new Intent(mActivity, VrCancelAnimationActivity.class)),
-                options);
+        Intent intent = mVrDaydreamApi.setupVrIntent(
+                new Intent(mActivity, VrCancelAnimationActivity.class));
+        // We don't want this to run in a new task stack, or we may end up resuming the wrong
+        // Activity when the VrCancelAnimationActivity finishes.
+        intent.setFlags(intent.getFlags() & ~Intent.FLAG_ACTIVITY_NEW_TASK);
+        mActivity.startActivity(intent, options);
         mNeedsAnimationCancel = false;
         return true;
     }