Roll pdfium, add GN build for pdf and pdfium.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 58f4623dc314243f5c8e13cb62a6f655761a5d6a..fdeda7ee7719dacf580a4dad54020a137b33dfa5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -47,6 +47,7 @@ group("root") {
     "//ipc",
     "//mojo",
     "//net",
+    "//pdf",
     "//ppapi:ppapi_c",
     "//printing",
     "//sandbox",
@@ -120,6 +121,7 @@ group("root") {
       "//content/public/renderer",
       "//dbus",
       "//extensions/common/api:extensions_api",
+      "//pdf",  # Not compiled on Android in GYP yet, either.
       "//ppapi:ppapi_c",
       "//third_party/libusb",
       # "//third_party/WebKit/Source/platform",