GN: Replace mojo/system.gni with forwarding groups

GN: Replace mojo/system.gni with forwarding groups

Many targets in mojo need to depend on different targets depending on if
they are building in the static or component build. In gyp, this is done
with variables (since that's the only thing you have) and the GN build
initially copied these variables into //mojo/system.gni. This worked, but
is not very idomatic in GN. This replaces the variables with groups that
forward to the correct depenedency depending on the build type and
target type (encoded in the target name).

R=brettw@chromium.org, viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/554363005

Cr-Commit-Position: refs/heads/master@{#295137}