Fixes GN boringssl deps path
Fixes GN boringssl deps path

The Mojo tree already DEPSes in boringssl into its //third_party. To
avoid DEPSing in two boringssl's, we need to look for the one there.
(The GN 'check' routine requires the sources to be found even if
noone depends on them.) Flutter will need to move its boringssl
DEPS from //dart/third_party/boringssl to //third_party/boringssl
to match.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1952383002 .

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk