Remove NativeThemeWin::PaintIndirect

We don't seem to actually use this complicated codepath in practice.

In particular:

1. // This block will only get hit with --enable-accelerated-drawing flag.
That flag is obsolete, and its replacement also seems to no longer exist.

2. // Scrollbar components on Windows Classic theme (on all Windows versions)
   // have particularly problematic alpha values, so always draw them
   // indirectly.
Turning on the Windows Classic theme on 64b Windows 7, I can't cause the block
to be hit today.

3. // In addition, scrollbar thumbs and grippers for the Windows XP
   // theme (available only on Windows XP) also need their alpha values
   // fixed.
We're no longer supporting XP and have been cleared to remove relevant code.

Once it's gone, so are several copies of functions that paint the
same UI element in different ways depending on whether they're drawing
to HDC or SkCanvas: Gutter, MenuSeparator, MenuBackground, and
MenuItemBackground.

R=pkasting@chromium.org
BUG=543755,579196,622692

Review-Url: https://codereview.chromium.org/2090003003
Cr-Commit-Position: refs/heads/master@{#404155}
2 files changed