Move declaration of is_fuzzer to where it is used

BUG=skia:6260

Change-Id: Iaca4fb1afb4d182f4d2ae64d939729363a226ce6
Reviewed-on: https://skia-review.googlesource.com/8772
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>

BUILD.gn

@@ -35,6 +35,7 @@ declare_args() {
   skia_enable_tools = is_skia_standalone
   skia_enable_vulkan_debug_layers = is_skia_standalone && is_debug
   skia_vulkan_sdk = getenv("VULKAN_SDK")
+  is_fuzzing = false
 }
 declare_args() {
   skia_use_dng_sdk =

gn/BUILDCONFIG.gn

@@ -17,7 +17,6 @@ declare_args() {
     ndk_api = 21
   }
   sanitize = ""
-  is_fuzzing = false
 }
 declare_args() {
   is_debug = !is_official_build