Fix regression where mouse hover effect is absent on the browser action chevron

It looks like a SetBorder(NULL) crept into
https://codereview.chromium.org/324393002, and the result is that the chevron
hover effect (i.e., draw a border) is overridden. Remove the call to
SetBorder().

BUG=407518

Review URL: https://codereview.chromium.org/492373004

Cr-Commit-Position: refs/heads/master@{#292158}

chrome/browser/ui/views/toolbar/browser_actions_container.cc

@@ -174,7 +174,6 @@ BrowserActionsContainer::BrowserActionsContainer(
     // the Chrome menu.
     if (!overflow_experiment) {
       chevron_ = new ChevronMenuButton(NULL, base::string16(), this, false);
-      chevron_->SetBorder(views::Border::NullBorder());
       chevron_->EnableCanvasFlippingForRTLUI(true);
       chevron_->SetAccessibleName(
           l10n_util::GetStringUTF16(IDS_ACCNAME_EXTENSIONS_CHEVRON));