Makes builds go faster by not having to re-run "gn gen" unnecessarily

Also adds a bunch of flags that configure uses.
  --max-load
  --max-jobs
  --extra-gn-args
  --depot-tools
  --bundled-win-toolchain

NOTRY=true
NOTREECHECKS=true

Change-Id: I6555623468d2b11d188ca29563586f5ea9b4dda9
Reviewed-on: https://chromium-review.googlesource.com/1016582
Commit-Queue: agrieve <agrieve@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52761}