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This change makes it easier to construct IconDescription by adding
default values to three out of four constructor params.

No functional change is intended, although on SK_ColorBLACK is changed
to gfx::kPlaceholderColor. This should not matter because all the icons
it applies to have hard-coded colors for every individual path (verified
manually).

Bug: none
Change-Id: Ieed356656b2e1f346cbecd76a4d230a445873e43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1985124
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728221}