Android: Discouraging comment on removing EmptyTabObserver

Added a comment in EmptyTabObserver against the future attempt to
replace EmptyTabObserver with TabObserver + default interface methods
as it has a side effect of increasing the number of generated methods.

Bug: 781359
Change-Id: Iea3a345c7cf522f69e9626245605a0f967af3bb4
Reviewed-on: https://chromium-review.googlesource.com/1155284
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580387}

chrome/android/java/src/org/chromium/chrome/browser/tab/EmptyTabObserver.java

@@ -14,6 +14,9 @@ import org.chromium.content_public.common.BrowserControlsState;
 
 /**
  * An implementation of the {@link TabObserver} which has empty implementations of all methods.
+ *
+ * Note: Do not replace this with TabObserver with default interface methods as it inadvertently
+ * bloats the number of methods. See https://crbug.com/781359.
  */
 public class EmptyTabObserver implements TabObserver {