bring back pdfium_test to all.gyp after rolling pdfium with win x64 fix

git-svn-id: http://src.chromium.org/svn/trunk/src/build@272868 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

all.gyp

@@ -100,6 +100,7 @@
             '../third_party/modp_b64/modp_b64.gyp:*',
             '../third_party/npapi/npapi.gyp:*',
             '../third_party/ots/ots.gyp:*',
+            '../third_party/pdfium/samples/samples.gyp:*',
             '../third_party/qcms/qcms.gyp:*',
             '../tools/gn/gn.gyp:*',
             '../tools/perf/clear_system_cache/clear_system_cache.gyp:*',