DevTools: generate_protocol_externs.js should accept both protocol files as arguments

Since protocol split, the generate_protocol_externs.js CLI has to accept
both protocol files as input.

BUG=none
R=dgozman

Review-Url: https://codereview.chromium.org/2158063002
Cr-Commit-Position: refs/heads/master@{#406052}

third_party/WebKit/Source/devtools/scripts/generate_protocol_externs.py

@@ -238,9 +238,10 @@ if __name__ == "__main__":
     import sys
     import os.path
     program_name = os.path.basename(__file__)
-    if len(sys.argv) < 4 or sys.argv[1] != "-o":
-        sys.stderr.write("Usage: %s -o OUTPUT_FILE INPUT_FILE\n" % program_name)
+    if len(sys.argv) < 5 or sys.argv[1] != "-o":
+        sys.stderr.write("Usage: %s -o OUTPUT_FILE INPUT_FILE_1 INPUT_FILE_2\n" % program_name)
         exit(1)
     output_path = sys.argv[2]
-    input_path = sys.argv[3]
-    generate_protocol_externs(output_path, input_path)
+    input_path_1 = sys.argv[3]
+    input_path_2 = sys.argv[4]
+    generate_protocol_externs(output_path, input_path_1, input_path_2)