Rename buildflag_headers for consistency

This updates ppapi.

Targets go from foo_features to foo_buildflags
And similarly the header now has buildflags in the name.

BUG=814485
TEST=covered by build
[email protected]

Change-Id: I25f990ebd1e7f8530ddd0cee24dba31c88a45fae
Reviewed-on: https://chromium-review.googlesource.com/980921
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Xiaohan Wang <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Commit-Queue: Scott Violet <[email protected]>
Cr-Commit-Position: refs/heads/master@{#546312}
diff --git a/content/BUILD.gn b/content/BUILD.gn
index 62be6fcc..e487443 100644
--- a/content/BUILD.gn
+++ b/content/BUILD.gn
@@ -4,7 +4,7 @@
 
 import("//tools/grit/grit_rule.gni")
 import("//build/config/nacl/config.gni")
-import("//ppapi/features/features.gni")
+import("//ppapi/buildflags/buildflags.gni")
 
 # Applied by targets internal to content.
 config("content_implementation") {