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/pdfium.gni b/pdfium.gni
index 8ce09f9..4548819 100644
--- a/pdfium.gni
+++ b/pdfium.gni
@@ -41,6 +41,9 @@
   # Build PDFium standalone
   pdf_is_standalone = false
 
+  # Build a complete static library
+  pdf_is_complete_lib = false
+
   # Enable coverage information
   use_coverage = false
 }