Silence deprecated warning for bison directive in blink's xpath_grammar.y

This commit adds -Wno-deprecated to the relevant target's cflags for
non-mac builds. If we ever update XCode's bison version to 2.7 or
better, we will be able to replace '%pure-parser' with
'%define api.pure full' in xpath_grammar.y.

Change-Id: Ib0390d7c91828c095a86eb07acb854f09fa13060
Bug: 1028421
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948245Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721573}

third_party/blink/renderer/core/xml/BUILD.gn

@@ -64,4 +64,11 @@ blink_core_sources("xml") {
     "xslt_unicode_sort.cc",
     "xslt_unicode_sort.h",
   ]
+
+  # XCode's ancient bison (v2.3 from 2006) prevents us from updating a
+  # deprecated bison directive. This will silence the warning.
+  # See https://crbug.com/1028421.
+  if (!is_mac) {
+    cflags = [ "-Wno-deprecated" ]
+  }
 }