Enable building pdf_use_skia in the standalone build.

This CL adds the necessary pieces so you can add pdf_use_skia=1 to GYP_DEFINES
and do a standalone build.

R=caryclark@google.com

Review URL: https://codereview.chromium.org/1812563002 .
diff --git a/.gitignore b/.gitignore
index cbd1e7d..45bcec5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,6 +8,7 @@
 /testing/gtest
 /third_party/llvm
 /third_party/llvm-build
+/third_party/skia
 /tools/clang
 /v8
 /xcodebuild