Add DCHECK to main menu builder.

The shift modifier is *almost* always incorrect to use for menu items -- it is
instead appropriate to apply the shift modifier directly to the keyEquivalent.

Bug: 882166
Change-Id: I6507a2a8981241cb1b80a8737213c5ac45fdab2e
Reviewed-on: https://chromium-review.googlesource.com/1259362Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596299}

chrome/browser/ui/cocoa/main_menu_builder.h

@@ -93,6 +93,9 @@ class MenuItemBuilder {
   // the one specified here is used instead.
   MenuItemBuilder& key_equivalent(NSString* key_equivalent,
                                   NSEventModifierFlags flags) {
+    DCHECK((flags & NSEventModifierFlagShift) == 0)
+        << "The shift modifier flag should be directly applied to the key "
+           "equivalent.";
     key_equivalent_ = key_equivalent;
     key_equivalent_flags_ = flags;
     return *this;