DevTools: prepare build config for externally hosted frontend

This change introduces the GN arg 'external_devtools_frontend', which
for now defaults to false. Follow up changes will implement the true case.

Bug: 1011259
Change-Id: I07afe85f1552516a02184c430fb6e54d3928f04b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838192
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704166}

third_party/blink/public/BUILD.gn

@@ -564,21 +564,27 @@ source_set("blink_headers") {
   ]
 }
 
+if (external_devtools_frontend) {
+  devtools_frontend_path = "//third_party/devtools-frontend"
+} else {
+  devtools_frontend_path = "//third_party/blink/renderer/devtools"
+}
+
 group("blink_devtools_frontend_resources") {
   public_deps = [
-    "//third_party/blink/renderer/devtools:devtools_frontend_resources",
+    "$devtools_frontend_path:devtools_frontend_resources",
   ]
 }
 
 group("blink_devtools_frontend_resources_files") {
   data_deps = [
-    "//third_party/blink/renderer/devtools:devtools_all_files",
+    "$devtools_frontend_path:devtools_all_files",
   ]
 }
 
 group("blink_generate_devtools_grd") {
   public_deps = [
-    "//third_party/blink/renderer/devtools:generate_devtools_grd",
+    "$devtools_frontend_path:generate_devtools_grd",
   ]
 }
 

third_party/blink/public/public_features.gni

@@ -10,6 +10,10 @@ declare_args() {
   # resources.pak. It is still possible to load JS files from disk by passing
   # --debug-devtools cmdline switch.
   debug_devtools = false
+
+  # If external_devtools_frontend is set to true, use DevTools frontend that is
+  # pulled as external dependency.
+  external_devtools_frontend = false
 }
 
 # Unhandled Tap enable means Contextual Search aka Tap to Search.