Move enable_pdf to a buildflag header.

chrome/common/BUILD.gn

Index: chrome/common/BUILD.gn
diff --git a/chrome/common/BUILD.gn b/chrome/common/BUILD.gn
index df8259b8b53a26838db6a37f24ba5c150ab66d71..95e78576e6f8dffc72a8f2213a0f7ba899af2b17 100644
--- a/chrome/common/BUILD.gn
+++ b/chrome/common/BUILD.gn
@@ -8,9 +8,14 @@ import("//chrome/common/features.gni")
 import("//chrome/process_version_rc_template.gni")  # For branding_file_path.
 import("//extensions/features/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//pdf/features.gni")
 import("//ppapi/features/features.gni")
 import("//tools/grit/grit_rule.gni")
 
+if (enable_pdf) {
+  assert(enable_plugins, "PDF support requires plugins be enabled.")
+}
+
 grit("resources") {
   source = "common_resources.grd"
   use_qualified_include = true
@@ -221,6 +226,7 @@ static_library("common") {
     "//mojo/edk/system",
     "//mojo/public/cpp/bindings",
     "//net",
+    "//pdf:features",
     "//ppapi/features",
     "//printing/features",
     "//skia",