Android: Fix NPE in Preview Tab

WebContentsObserver.didFinishNavigation that PreviewTab is listening
to can come before its views are ready. This CL stops updating
caption in such case to prevent NPE reported in the bug.

Bug: 1020888
Change-Id: I9240e32015e0df631c3b80546c99d8e9e0a97c5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898152Reviewed-by: Donn Denman <donnd@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712408}

chrome/android/java/src/org/chromium/chrome/browser/compositor/bottombar/ephemeraltab/EphemeralTabCaptionControl.java

@@ -147,6 +147,7 @@ public class EphemeralTabCaptionControl extends OverlayPanelTextViewInflater {
      * @param text String to use for caption.
      */
     public void setCaptionText(String text) {
+        if (mCaption == null) return;
         mCaption.setText(text);
         invalidate();
     }