Use correct relative dir for Chromium compilations

In chromium, DevTools lives in third_party/devtools-frontend/src/.
Since root_gen_dir is equivalent to chromium/src/gen, the rebasing of
the files in the expected JSON files would be incorrect. This worked
fine in DevTools standalone, as the root_gen_dir === target_gen_dir.

[email protected],[email protected]

Bug: 1174013
Change-Id: I95ba7301aea7147913f9faa8c2ed5d6593684a92
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2846329
Commit-Queue: Tim van der Lippe <[email protected]>
Commit-Queue: Paul Lewis <[email protected]>
Commit-Queue: Kim-Anh Tran <[email protected]>
Auto-Submit: Tim van der Lippe <[email protected]>
Reviewed-by: Paul Lewis <[email protected]>
Reviewed-by: Kim-Anh Tran <[email protected]>
diff --git a/BUILD.gn b/BUILD.gn
index d6f599d..d6cddb4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -205,7 +205,7 @@
   data_keys = [ "grd_files" ]
 
   output_conversion = "json"
-  rebase = root_gen_dir
+  rebase = target_gen_dir
 
   deps = [
     ":build_release_devtools",