Disable WebAssembly in V8 for standalone PDFium.

PDFs only need to run JavaScript. Thus PDFium does not need V8's
WebAssembly capabilities. Do so by setting v8_enable_webassembly = false
in PDFium's //.gn file. This does not V8 in embedders like Chromium,
where Chomium's //.gn file takes precedence.

Change-Id: I49c5c0786bf3470412eb39e050fb380b8fd796e8
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/84850
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

.gn

diff --git a/.gn b/.gn
index 4cbf594..787ba75 100644
--- a/.gn
+++ b/.gn
@@ -8,6 +8,9 @@
   v8_extra_library_files = []
   v8_experimental_extra_library_files = []
 
+  # PDFs only need to run JavaScript.
+  v8_enable_webassembly = false
+
   # Turns on compiler optimizations in V8 in Debug build.
   v8_optimized_debug = true
 }