[Win] The tab strip's opaque background overlaps the avatar button.

The tab strip is actually about 4 px taller than the drawn tab, so when
applying an alpha to make the inactive tabs slightly transparent, this
alpha "leaks" onto the avatar button and adds a transparency to its
bottom border.

I set the background to red to demonstrate this, so here are some screenshots:
Before the CL:
https://drive.google.com/open?id=0B1B1Up4p2NRMRXRJRjN2MG41Y0k&authuser=1

After the CL:
https://drive.google.com/open?id=0B1B1Up4p2NRMRzJJUmluT2QwSDA&authuser=1

Finally, how the tab strip actually looks like with normal colours:
Before the CL:
https://drive.google.com/open?id=0B1B1Up4p2NRMd0F0dWZnby12T0k&authuser=1

After the CL:
https://drive.google.com/open?id=0B1B1Up4p2NRMa2o0aUNxaE12ZGs&authuser=1

BUG=430380
TEST=Start Chrome with --enable-new-avatar-menu. The bottom of the avatar
button should look the same throughout (and not have any random transparencies)

Review URL: https://codereview.chromium.org/859303008

Cr-Commit-Position: refs/heads/master@{#312721}

1 files changed, 9 insertions, 1 deletions

diff --git a/chrome/browser/ui/views/tabs/tab_strip.cc b/chrome/browser/ui/views/tabs/tab_strip.cc
index 0fa4752..f35c59c 100644
--- a/chrome/browser/ui/views/tabs/tab_strip.cc
+++ b/chrome/browser/ui/views/tabs/tab_strip.cc
@@ -1296,8 +1296,16 @@ void TabStrip::PaintChildren(gfx::Canvas* canvas,
     paint.setColor(SkColorSetARGB(alpha, 255, 255, 255));
     paint.setXfermodeMode(SkXfermode::kDstIn_Mode);
     paint.setStyle(SkPaint::kFill_Style);
+
+    // The tab graphics include some shadows at the top, so the actual
+    // tabstrip top is 4 px. above the apparent top of the tab, to provide room
+    // to draw these. Exclude this region when trying to make tabs transparent
+    // as it's transparent enough already, and drawing in this region can
+    // overlap the avatar button, leading to visual artifacts.
+    const int kTopOffset = 4;
     // The tabstrip area overlaps the toolbar area by 2 px.
-    canvas->DrawRect(gfx::Rect(0, 0, width(), height() - 2), paint);
+    canvas->DrawRect(
+        gfx::Rect(0, kTopOffset, width(), height() - kTopOffset - 2), paint);
   }
 
   // Now selected but not active. We don't want these dimmed if using native