Move Skia GN sources to a separate .gni file.

Move Skia GN sources to a separate .gni file.

This file will be imported by Chrome to access the sources lists.

Once Chrome is updated to use this file, changes to the skia .gypi layout can
be done entirely within the skia repository as long as the resulting lists
produced by the new .gni file have the same name.

Marks skia_for_chromium_defines as obsolete and moves the definition into the new .gni file. We can remove the .gypi file when Chrome is updated.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2302803005

Review-Url: https://codereview.chromium.org/2302803005