Remove webkit dependency in grit_rule.

We needed to set a grit define to build the blink_resources target
properly. We were doing this via a global define that affected all
targets, but now we're doing this with a local define instead.

R=rtoy@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1338193003

Cr-Commit-Position: refs/heads/master@{#348812}

build/secondary/tools/grit/grit_rule.gni

@@ -79,7 +79,6 @@
 #     # files.
 #   }
 import("//build/config/chrome_build.gni")
-import("//third_party/WebKit/Source/config.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
@@ -251,13 +250,6 @@ if (enable_google_now) {
   ]
 }
 
-if (use_concatenated_impulse_responses) {
-  grit_defines += [
-    "-D",
-    "use_concatenated_impulse_responses",
-  ]
-}
-
 if (enable_webrtc) {
   grit_defines += [
     "-D",