Fix shelf view unit test with UnifiedSystemTray.

ShelfViewTest's way of triggering overflow button in Shelf sometimes
didn't work depending on the width of the StatusArea. It led to test
failures with a launching flag which changes Shelf width.

Failing tests (with --enable-features=SystemTrayUnified):
OverflowButtonActiveInkDropTest.TouchDragOut
OverflowButtonActiveInkDropTest.TouchDragOutAndBack
OverflowButtonInkDropTest.TouchDragOut
OverflowButtonInkDropTest.TouchDragOutAndBack

TEST=ash_unittests --enable-features=SystemTrayUnified
BUG=847104

Change-Id: I00f64255936ca32daba375832c0b39ccc3e27014
Reviewed-on: https://chromium-review.googlesource.com/1088532
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564938}

ash/shelf/shelf_view_unittest.cc

@@ -264,7 +264,7 @@ class ShelfViewTest : public AshTestBase {
     NotificationTray::DisableAnimationsForTest(true);
 
     // The bounds should be big enough for 4 buttons + overflow chevron.
-    shelf_view_->SetBounds(0, 0, 500, kShelfSize);
+    ASSERT_GE(shelf_view_->width(), 500);
 
     test_api_.reset(new ShelfViewTestAPI(shelf_view_));
     test_api_->SetAnimationDuration(1);  // Speeds up animation for test.