Move enable_pdf to a buildflag header.

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 4b75bf6e9bf27c5ab67b18a7d431efebf003f91a..6bde637ef354b42e4f2b4fdcc69e01d761e7f54f 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -130,6 +130,7 @@ static_library("test_support") {
     "//media:media_features",
     "//net",
     "//net:test_support",
+    "//pdf",
     "//ppapi/features",
     "//printing/features",
     "//skia",
@@ -227,10 +228,6 @@ static_library("test_support") {
     ]
   }
 
-  if (enable_plugins && enable_pdf) {
-    public_deps += [ "//pdf" ]
-  }
-
   if (use_ash) {
     sources += [
       "base/default_ash_event_generator_delegate.cc",