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-Commit-Position: refs/heads/master@{#438441}