[Merge to M-41 2272]: views::Label: Invalidate cached draw flags when…

… background color is changed

>Transparency of the background color affects the result of |Label::ComputeDrawStringFlags()|.
>
>BUG=447932
>R=sky
>NOTRY=true
>
>Review URL: https://codereview.chromium.org/846853003
>
>Cr-Commit-Position: refs/heads/master@{#311088}

BUG=447932
TBR=sky

Review URL: https://codereview.chromium.org/816223004

Cr-Commit-Position: refs/branch-heads/2272@{#9}
Cr-Branched-From: 827a380-refs/heads/master@{#310958}

ui/views/controls/label.cc

@@ -108,6 +108,7 @@ void Label::SetBackgroundColor(SkColor color) {
   background_color_ = color;
   background_color_set_ = true;
   RecalculateColors();
+  cached_draw_params_.text.clear();
 }
 
 void Label::SetShadows(const gfx::ShadowValues& shadows) {