Roll build, buildtools, and V8

Unlanded attempt:
https://pdfium-review.googlesource.com/c/2410/

BUG=690088

Change-Id: I0bd3396426bd9faadb8c8dba2c2b7a7b9b52b178
Reviewed-on: https://pdfium-review.googlesource.com/2576
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>
diff --git a/.gn b/.gn
index fbcbf4b..ce52529 100644
--- a/.gn
+++ b/.gn
@@ -4,3 +4,12 @@
 
 buildconfig = "//build/config/BUILDCONFIG.gn"
 secondary_source = "//build/secondary/"
+
+default_args = {
+  v8_extra_library_files = []
+  v8_experimental_extra_library_files = []
+  v8_enable_inspector = false
+
+  # Turns on compiler optimizations in V8 in Debug build.
+  v8_optimized_debug = true
+}