[Dice] Add accessible name to dropdown arrow

This CL adds an accessible name to the
"Sync with another account" dropdown arrow in
the contextual promos.

Bug: 814986
Change-Id: Icb43e48a398178d30cd7517b1aba2c1efa697492
Reviewed-on: https://chromium-review.googlesource.com/939397Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539822}

chrome/browser/ui/views/sync/dice_signin_button.cc

@@ -109,6 +109,8 @@ DiceSigninButton::DiceSigninButton(const AccountInfo& account,
         views::Button::STATE_NORMAL,
         gfx::CreateVectorIcon(kSigninButtonDropDownArrowIcon,
                               kDropDownArrowIconSize, SK_ColorWHITE));
+    arrow_->SetAccessibleName(l10n_util::GetStringUTF16(
+        IDS_PROFILES_DICE_SIGNIN_WITH_ANOTHER_ACCOUNT_BUTTON));
     AddChildView(arrow_);
   }