GN: Add assert that //pdf is referenced only when enable_pdf=true

BUG=

Review URL: https://codereview.chromium.org/1548943002

Cr-Commit-Position: refs/heads/master@{#366981}

pdf/BUILD.gn

@@ -2,8 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/features.gni")
 import("//third_party/pdfium/pdfium.gni")
 
+assert(enable_pdf)
+
 pdf_engine = 0  # 0 PDFium
 
 static_library("pdf") {