Fix BUILD.gn to load the correct V8 snapshot configuration

BUG=pdfium:299
R=kcc@chromium.org

Review URL: https://codereview.chromium.org/1520443002 .
diff --git a/BUILD.gn b/BUILD.gn
index f1583ab..3b029b5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
@@ -170,6 +171,7 @@
       "//v8",
       "//v8/include",
     ]
+    configs += [ "//v8:external_startup_data" ]
   }
   configs += [ ":pdfium_config" ]
 }
@@ -688,6 +690,7 @@
     public_deps = [
       "//v8",
     ]
+    configs += [ "//v8:external_startup_data" ]
   } else {
     sources += [ "fpdfsdk/src/javascript/JS_Runtime_Stub.cpp" ]
   }
@@ -780,6 +783,7 @@
       "//v8",
       "//v8/include",
     ]
+    configs += [ "//v8:external_startup_data" ]
   }
   configs += [ ":pdfium_config" ]
 }
diff --git a/samples/BUILD.gn b/samples/BUILD.gn
index a66178a..a214f1b 100644
--- a/samples/BUILD.gn
+++ b/samples/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
+import("//build_overrides/v8.gni")
 import("../pdfium.gni")
 
 group("samples") {
@@ -48,6 +49,7 @@
       "//v8",
       "//v8/include",
     ]
+    configs += [ "//v8:external_startup_data" ]
   }
   configs += [ ":pdfium_samples_config" ]
 }