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For resources not provided by the native theme (i.e. GTK2UI), fall
back to desktop (not ash) resources by calling MapThemeImage in this
case.

BUG=383711
R=erg@chromium.org
TEST=The shadow running along the top edge of the address bar to the
right of the tabs is visible again, especially when using a bright GTK
theme like Adwaita or Radiance. (see crbug.com/383711)

Review URL: https://codereview.chromium.org/1243513002

Cr-Commit-Position: refs/heads/master@{#339744}