[Paint Preview] Don't exit player on same document navigation

During capture TabObserver#onLoadStarted() may be triggered if the page
is still loading (e.g lazily loaded content). We should avoid exiting
in such a scenario.

Bug: 1084657
Change-Id: I680c85d6b918bf054e285f37445a7c3674158959
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212402Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771623}

chrome/android/java/src/org/chromium/chrome/browser/paint_preview/PaintPreviewTabHelper.java

@@ -59,6 +59,9 @@ public class PaintPreviewTabHelper extends EmptyTabObserver implements UserData
 
     @Override
     public void onLoadStarted(Tab tab, boolean toDifferentDocument) {
+        // Note this doesn't apply to forced reloads, which is desirable in this case.
+        if (!toDifferentDocument) return;
+
         removePaintPreviewDemoIfShowing();
     }