webkit: Catch OSError for when gperf isn't avaliable.

third_party/WebKit/Source/build/scripts/make_css_value_keywords.py

Index: third_party/WebKit/Source/build/scripts/make_css_value_keywords.py
diff --git a/third_party/WebKit/Source/build/scripts/make_css_value_keywords.py b/third_party/WebKit/Source/build/scripts/make_css_value_keywords.py
index f86922f76afad5f783c9b7826656c850b36aed58..c39804676260e4c1f96f6541af64b3f914b10aa3 100755
--- a/third_party/WebKit/Source/build/scripts/make_css_value_keywords.py
+++ b/third_party/WebKit/Source/build/scripts/make_css_value_keywords.py
@@ -165,8 +165,16 @@ class CSSValueKeywordsWriter(json5_generator.Writer):
         gperf_args = [self.gperf_path, '--key-positions=*', '-P', '-n']
         gperf_args.extend(['-m', '50'])  # Pick best of 50 attempts.
         gperf_args.append('-D')  # Allow duplicate hashes -> More compact code.
-        gperf = subprocess.Popen(gperf_args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, universal_newlines=True)
-        return gperf.communicate(gperf_input)[0]
+
+        # If gperf isn't in the path we get an OSError. We don't want to use
+        # the normal solution of shell=True (as this has to run on many
+        # platforms), so instead we catch the error and raise a
+        # CalledProcessError like subprocess would do when shell=True is set.
+        try:
+            gperf = subprocess.Popen(gperf_args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, universal_newlines=True)
+            return gperf.communicate(gperf_input)[0]
+        except OSError:
+            raise subprocess.CalledProcessError(127, gperf_args, output='Command not found.')
 
 
 if __name__ == "__main__":