[CFI] Add entry to CFI blacklist

This cfi-icall failure manifests with new versions of libgtk, discovered
after an upgrade.

Change-Id: I9f5d90c55599d4b935b0ddd3184df203bc674767
Reviewed-on: https://chromium-review.googlesource.com/794872Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519893}

tools/cfi/blacklist.txt

@@ -175,6 +175,9 @@ fun:*CertVerifyProcNSS*VerifyInternalImpl*
 # third_party/skia/include/gpu/gl/GrGLFunctions.h
 fun:*GrGLFunction*
 
+# chrome/browser/ui/libgtkui/gtk_util.cc
+fun:*AppendCssNodeToStyleContext*
+
 ######### Function pointers cast to incorrect type signatures
 
 # libicu is currently compiled such that in libicu the 'UChar' type is a