[omnibox] Restrict tab switch button space activation to no modifiers

Currently one can activate the tab switch button with shift-space etc.
This CL restricts it to only space, no modifiers.

Bug: 780835
Change-Id: I49b50d9d693e6201d7c3e74c91929bf71280865f
Reviewed-on: https://chromium-review.googlesource.com/1163991Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Kevin Bailey <krb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581122}

chrome/browser/ui/views/omnibox/omnibox_view_views.cc

@@ -1328,6 +1328,7 @@ bool OmniboxViewViews::HandleKeyEvent(views::Textfield* textfield,
       break;
 
     case ui::VKEY_SPACE:
+      if (!(control || alt || shift))
       {
         size_t start, end;
         GetSelectionBounds(&start, &end);