Move enable_pdf to a buildflag header.

Removes the build flag from //build/config and moves it to a new .gni file in
//pdf. Converts the define to a BUILDFLAG.

Makes it possible to unconditionally depend on "//pdf" without checking the
enable_pdf flag. This cleans up most of the callers. There is still a
dependency on plugins and this assertion is moved to the top of
//chrome/common/BUILD.gn.

The assertion for the printing dependency on PDF is moved to the
//printing/BUILD.gn so the enable_pdf flag isn't injected everywhere
enable_printing is needed.

Review-Url: https://codereview.chromium.org/2576573002
Cr-Original-Commit-Position: refs/heads/master@{#438441}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e5cc5c689fef092ac8b0f9bf91caeff492c4e857

config/BUILD.gn

@@ -58,9 +58,6 @@ declare_args() {
 config("feature_flags") {
   # Don't use deprecated V8 APIs anywhere.
   defines = [ "V8_DEPRECATION_WARNINGS" ]
-  if (enable_pdf) {
-    defines += [ "ENABLE_PDF=1" ]
-  }
   if (dcheck_always_on) {
     defines += [ "DCHECK_ALWAYS_ON=1" ]
   }

config/features.gni

@@ -21,8 +21,6 @@ if (is_android) {
 }
 
 declare_args() {
-  enable_pdf = !is_android && !is_ios && !is_chromecast
-
   # Enables Native Client support.
   # Temporarily disable nacl on arm64 linux to get rid of compilation errors.
   # TODO(mcgrathr): When mipsel-nacl-clang is available, drop the exclusion.