GitLab

Adds a toolchain variable "concurrent_links". The value of this variable in the default toolchain is used as the depth of the link pool for the build.

Remove the "pool" stuff from the tool definitions that was never hooked up. The link pool is hardcoded in GN and is controlled by the above variable.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/508763002

Cr-Commit-Position: refs/heads/master@{#292261}