Update GN target for angle_unittests in the gn_isolate map.

In the GN builds, we need to maintain a mapping from ninja
build target ("angle_unittests") to the GN build label
used to build the targets, since the way we figure out the
runtime deps needed for the isolate is keyed off of the build label.

When we updated the angle_unittests location, we failed to update
the mapping.

TBR=cwallez@chromium.org, phajdan.jr@chromium.org
BUG=519778

Review URL: https://codereview.chromium.org/1323293006

Cr-Commit-Position: refs/heads/master@{#347299}
1 file changed