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Adds the initial plumbing needed in order to move
Ash-specific .icon files out of ui/gfx/vector_icons
and into ash/common/resources/vector_icons. As a
result, these icons will only be included when the
'ash' component is built.

Previously, the icon file foo_bar.icon would be
identified in code as the enum value
gfx::VectorIconId::FOO_BAR. For Ash-specific icons,
foo_bar.icon is now identified in code as the
constant ash::kFooBarIcon, which maps to a
VectorIcon struct (also introduced in this CL).
The eventual goal is for all icons to be identified
in this manner, and for the gfx::VectorIconId struct
to be removed.

BUG=626786
TEST=manual

Review-Url: https://codereview.chromium.org/2251643011
Cr-Commit-Position: refs/heads/master@{#415782}