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arrow that is used in layout tests. This modification allows the
arrow to look the right size in different device scale factors when
using zoom-for-dsf. This is achieved by using a square drop down
arrow instead of a rectangular one. This logic is similar to what
the native theme engine uses.

BUG=640256
COMPONENT=Test Runners, Mock Web Theme Engine
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2340633002
Cr-Commit-Position: refs/heads/master@{#419930}