[Extensions] Make extension actions use gfx::Image over gfx::ImageSkia.

gfx::Images cache different representations (such as NSImages), whereas
gfx::ImageSkias (as a representation themselves) do not. Prefer to use
gfx::Image in most icon code, so that these representations are cached.

BUG=452971

Review URL: https://codereview.chromium.org/882243002

Cr-Commit-Position: refs/heads/master@{#313768}
11 files changed