Split out blimp client build config to its own config.gni file

blimp/client/core/config.gni

Index: blimp/client/core/config.gni
diff --git a/third_party/leakcanary/config.gni b/blimp/client/core/config.gni
similarity index 50%
copy from third_party/leakcanary/config.gni
copy to blimp/client/core/config.gni
index f6adcddf3ecc4442c7793666dfeecaf1f108fea8..53e5d804eeb1a672910f10f307c02243d5d31fb2 100644
--- a/third_party/leakcanary/config.gni
+++ b/blimp/client/core/config.gni
@@ -3,5 +3,8 @@
 # found in the LICENSE file.
 
 declare_args() {
-  enable_leakcanary = false
+  # Enables the blimp client. This is required to get the actual implementation
+  # of blimp.
+  # By default, only a dummy-implementation is provided.
+  enable_blimp_client = false
 }