Move test runner to a component

BUG=478250
R=mkwst@chromium.org,torne@chromium.org,jam@chromium.org

Review URL: https://codereview.chromium.org/1167703002

Cr-Commit-Position: refs/heads/master@{#332583}

diff --git a/BUILD.gn b/BUILD.gn
index 224591f..0f946dc 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -748,8 +748,8 @@
 
     if (is_win) {
       deps += [
+        "//components/test_runner:layout_test_helper",
         "//content/shell:crash_service",
-        "//content/shell:layout_test_helper",
       ]
     }
 
@@ -760,7 +760,7 @@
     if (is_mac) {
       deps += [
         "//breakpad:dump_syms($host_toolchain)",
-        "//content/shell:layout_test_helper",
+        "//components/test_runner:layout_test_helper",
       ]
     }