Give PDFium's top-level build a "default" group

Limit stray work when developers neglect to specify a target. This
should only affect standalone PDFium builds where PDFium's BUILD.gn
is the top-level build file.

See $ gn help ninja_rules

Bug: pdfium:1881
Change-Id: If885a009be91962abfd95f343df3cb2137e0646a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/97010
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 0cc37ba..ded6132 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -6,6 +6,11 @@
 import("//testing/test.gni")
 import("pdfium.gni")
 
+group("default") {
+  testonly = true
+  deps = [ ":pdfium_all" ]
+}
+
 group("freetype_common") {
   public_deps = []
   if (pdf_bundle_freetype) {