GitLab

Moves (some of) gyp environment setup out of gyp_chromium into separate
module, and shares that between gyp_chromium and landmines.py.

landmines.py is added as the first entry in DEPS hooks so that it can
clobber the entire build directory before running other hooks that
extract/generate into the build dir.

Reland with fix for ios, and for clean pull.

R=iannucci@chromium.org
BUG=400011

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=289099

Review URL: https://codereview.chromium.org/457003004

Cr-Commit-Position: refs/heads/master@{#289546}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289546 0039d316-1c4b-4281-b951-d872f2087c98