GN: Allow XCode projects to use ICECC environment variables.

Without them XCode cannot compile generated projects with ICECC.

Bug: 831500
Change-Id: I3b50c730660c4e8a6a06b9fda340e20edb87e40a
Reviewed-on: https://chromium-review.googlesource.com/1006577Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Michal Pichlinski <mpichlinski@opera.com>
Cr-Commit-Position: refs/heads/master@{#550159}

tools/gn/xcode_writer.cc

@@ -48,9 +48,13 @@ struct SafeEnvironmentVariableInfo {
 };
 
 SafeEnvironmentVariableInfo kSafeEnvironmentVariables[] = {
-    {"HOME", true}, {"LANG", true},    {"PATH", true},
-    {"USER", true}, {"TMPDIR", false},
-};
+    {"HOME", true},
+    {"LANG", true},
+    {"PATH", true},
+    {"USER", true},
+    {"TMPDIR", false},
+    {"ICECC_VERSION", true},
+    {"ICECC_CLANG_REMOTE_CPP", true}};
 
 XcodeWriter::TargetOsType GetTargetOs(const Args& args) {
   const Value* target_os_value = args.GetArgOverride(variables::kTargetOs);