Normalize generated file paths to match main tree.

Previously dawn_native files were in src/dawn_native/ while generated
files were in dawn_native/. This makes some things complicated when
integrating in other source trees so normalize all generated files to be
in paths that match the main tree.

BUG=dawn:225

Change-Id: I5b3e04d37a16251143578dfb7a31445b229fe4ac
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/11300
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/common/BUILD.gn

@@ -31,13 +31,16 @@ if (build_with_chromium) {
 
 config("dawn_public_include_dirs") {
   include_dirs = [
-    "${target_gen_dir}/../..",
+    "${target_gen_dir}/../../src/include",
     "${dawn_root}/src/include",
   ]
 }
 
 config("dawn_internal") {
-  include_dirs = [ "${dawn_root}/src" ]
+  include_dirs = [
+    "${target_gen_dir}/../../src",
+    "${dawn_root}/src",
+  ]
 
   defines = []
   if (dawn_always_assert || dcheck_always_on || is_debug) {