Modifies the logic to compute the dimensions of the drop down menu

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}