Added notification when user presses backspace to use new shortcut.

chrome/browser/ui/browser_command_controller.cc

Index: chrome/browser/ui/browser_command_controller.cc
diff --git a/chrome/browser/ui/browser_command_controller.cc b/chrome/browser/ui/browser_command_controller.cc
index 87f1fe6275bcae838258c55855a4f0be5d2d10a0..5d97b8ff461ae14572d5f7b648dc4b21dd149d89 100644
--- a/chrome/browser/ui/browser_command_controller.cc
+++ b/chrome/browser/ui/browser_command_controller.cc
@@ -321,14 +321,20 @@ void BrowserCommandController::ExecuteCommandWithDisposition(
   switch (id) {
     // Navigation commands
     case IDC_BACKSPACE_BACK:
-      if (!base::FeatureList::IsEnabled(kBackspaceGoesBackFeature))
+      if (!base::FeatureList::IsEnabled(kBackspaceGoesBackFeature)) {
+        browser_->window()->ShowNewBackShortcutBubble(false);

[Matt Giuca, 2016/05/23 04:22:57]

As noted on https://codereview.chromium.org/1992003002/, I think it's quite
confusing to have a conditional fall-through like this, but I'm preserving that
logic here.

I'd also be happy to change it back to
if (feature enabled)
  GoBack()
else
  ShowBackspaceNewShortcutBubble()

[Peter Kasting, 2016/05/24 23:35:08]

I think what you've done is fine.

         break;
+      }
+      // FALL THROUGH
     case IDC_BACK:
       GoBack(browser_, disposition);
       break;
     case IDC_BACKSPACE_FORWARD:
-      if (!base::FeatureList::IsEnabled(kBackspaceGoesBackFeature))
+      if (!base::FeatureList::IsEnabled(kBackspaceGoesBackFeature)) {
+        browser_->window()->ShowNewBackShortcutBubble(true);
         break;
+      }
+      // FALL THROUGH
     case IDC_FORWARD:
       GoForward(browser_, disposition);
       break;