Harmonize paths to dependent targets.

This CL consistently use:
 * relative paths for WebRTC dependent targets (test_support)
 * absolute paths for shared dependent targets (abseil)
This is a necessary (but insufficient) step to build WebRTC tests
from Chromium tree (rtc_include_tests=true), since test/ doesn't
sit anymore in the top level directory.

We also make sure that target declarations and uses are
consistent in regard to build_with_chromium flag.

Bug: webrtc:9943
Bug: webrtc:9855
Change-Id: I21dea98894df2fd4bfe2fd7ee7b71ba971e0ab5b
Reviewed-on: https://webrtc-review.googlesource.com/c/108720
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25445}
diff --git a/audio/BUILD.gn b/audio/BUILD.gn
index d569d27..b44bbe5 100644
--- a/audio/BUILD.gn
+++ b/audio/BUILD.gn
@@ -278,7 +278,7 @@
       "../test:single_threaded_task_queue",
       "../test:test_common",
       "../test:test_main",
-      "//test:test_support",
+      "../test:test_support",
       "//testing/gtest",
       "//third_party/abseil-cpp/absl/memory",
     ]