[ios] Made IDC_BACK and IDC_FORWARD safe.

NavigationManager's GoBack() and GoForward() crash if navigation is not
possible. This mirrors NavigationController's behavior so
NavigationManager should only be changed along with
NavigationController.

KeyCommandsProvider sends IDC_FORWARD and IDC_BACK commands without
checking that navigation is possible and app crashes.

There are 2 fully correct options to fix this bug:
1.) Change NavigationManager and NavigationController, so they don't
    crash (which has both pros and cons).
2.) Change KeyCommandsProvider, so it does not send IDC_FORWARD,
    IDC_BACK commands if navigation is not possible.

Correct fix will be tracked in a separate bug (crbug.com/677160), and
this CL simply fixes a crash in the most simplest way w/o touching
Tab class (which will be removed).

BUG=676727

Review-Url: https://codereview.chromium.org/2605713002
Cr-Commit-Position: refs/heads/master@{#440773}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -3872,7 +3872,10 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
 
   switch (command) {
     case IDC_BACK:
-      [[_model currentTab] goBack];
+      // TODO(crbug.com.677160): Remove |canGoBack| check.
+      if ([_model currentTab].canGoBack) {
+        [[_model currentTab] goBack];
+      }
       break;
     case IDC_BOOKMARK_PAGE:
       [self initializeBookmarkInteractionController];
@@ -3913,7 +3916,10 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
       [self searchFindInPage];
       break;
     case IDC_FORWARD:
-      [[_model currentTab] goForward];
+      // TODO(crbug.com.677160): Remove |canGoForward| check.
+      if ([_model currentTab].canGoForward) {
+        [[_model currentTab] goForward];
+      }
       break;
     case IDC_FULLSCREEN:
       NOTIMPLEMENTED();