[bfcache] Add a comment to ContentTranslateDriver.

At the moment ContentTranslateDriver disables all translates
for back-forward navigations. Add a comment saying that it should
stay so for pages which are restored from back-forward cache
even when the rest of history navigations will be translated again.

R=hajimehoshi@chromium.org
CC=bfcache-bugs@chromium.org
BUG=1001087

Change-Id: I19b17ff9958984aa903ea14a421a02693694bf5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836193
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702375}

components/translate/content/browser/content_translate_driver.cc

@@ -243,6 +243,11 @@ void ContentTranslateDriver::NavigationEntryCommitted(
     // Workaround for http://crbug.com/653051: back navigation sometimes have
     // the reload core type. Once http://crbug.com/669008 got resolved, we
     // could revisit here for a thorough solution.
+    //
+    // This means that the new translation won't be started when the page
+    // is restored from back-forward cache, which is the right thing to do.
+    // TODO(crbug.com/1001087): Ensure that it stays disabled for
+    // back-forward navigations even when bug above is fixed.
     return;
   }