[Merge][Win] The tab strip's opaque background overlaps the avatar button.
[Merge][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
TBR=noms@chromium.org
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}
(cherry picked from commit 298156be041512f6d4a78bdba5c90454d3d2dfdb)

Review URL: https://codereview.chromium.org/882723004

Cr-Commit-Position: 665a5787e4610f1ec55d3a4e2f02ab0724e38987@{#135}
Cr-Branched-From: 4cd5f18d847990599feba509541d8a7b9a16883a@{#310958}

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