Fuchsia: Reland "Relocate fidl_library.gni to build/config/fuchsia."

fidl_library.gni will be used for targets defined outside "third_party",
so promoting the GN template to a higher level makes sense.

The relocation was verified using "gn gen --check".

TBR=sergeyu@chromium.org

Change-Id: Ib632e466185f77f7b4f5bc43b13439bdd28faf3c
Reviewed-on: https://chromium-review.googlesource.com/1091157Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565376}

base/BUILD.gn

@@ -68,7 +68,7 @@ if (is_android) {
 }
 
 if (is_fuchsia) {
-  import("//third_party/fuchsia-sdk/fidl_library.gni")
+  import("//build/config/fuchsia/fidl_library.gni")
 }
 
 config("base_flags") {

third_party/fuchsia-sdk/fuchsia_sdk_pkg.gni

@@ -4,7 +4,7 @@
 
 assert(is_fuchsia)
 
-import("fidl_library.gni")
+import("//build/config/fuchsia/fidl_library.gni")
 
 # Templates for Fuchsia SDK packages.
 

webrunner/BUILD.gn

@@ -4,8 +4,8 @@
 
 assert(is_fuchsia)
 
+import("//build/config/fuchsia/fidl_library.gni")
 import("//build/config/fuchsia/rules.gni")
-import("//third_party/fuchsia-sdk/fidl_library.gni")
 import("//tools/grit/repack.gni")
 
 executable("webrunner") {