Allow adding complete_static_lib to build

The complete_static_lib will cause all of the various
libraries generated during the build to get bundled into
a single libpdfium.a. This makes linking a lot easier
for external applications as they just -lpdfium instead
of trying to get the correct link order.

See https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#complete_static_lib
for more information.

Note, this just does pdfium at the moment, the V8 and
XFA libs will need to get changed in the future if they're
needed.

BUG=pdfium:451

Change-Id: Id1f2d999652e2f951bcd77591f2514641371324e
Reviewed-on: https://pdfium-review.googlesource.com/3220
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 5de9d19..58798d9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -214,6 +214,10 @@
   } else {
     libs += [ "freetype" ]
   }
+
+  if (pdf_is_complete_lib) {
+    complete_static_lib = true
+  }
 }
 
 static_library("test_support") {