This ports logic from gyp_chromium to skip gyp by environment
variable. The variable name stays the same for chromium
and v8 as it is shared across other infra tools.

BUG=chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2049683004
Cr-Commit-Position: refs/heads/master@{#36860}