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This is a regression by crbug.com/432795.  Scrollbar thickness is fixed regardless
of zoom level.  So popup width was too narrow in less-than-100% zoom level.

With this CL, popupInternalPaddingEnd() returns zoomed value as ever if zoom level
is 100%+, and returns a value based on actual scrollbar thickness otherwise.

Also, popupIntenalPaddingEnd() respects to the actual scrollbar thickness instead
of returning fixed '18' pixel.  The default scrollbar thickness on Windows is 17.
popupInternalPaddingEnd() returns 1 + <scrollbar thickness>.

We need to update ThemePainterDefault::setupMenuListArrow() so that it can support
variable width of scrollbars.

Summary of behavior changes:

All platforms except Mac:
  menulist box is wider in less-than-100% zoom level.

Windows Aura theme:
  menulist box is not changed in 100%+ zoom level.

Non-Windows Aura theme and Mock theme:
  menulist box  is narrower by 2px because scrollbar thickness is 15px.

BUG=667236
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2560733002
Cr-Commit-Position: refs/heads/master@{#437864}