Dynamically generate dependencies of browserify build actions

Instead of having to list in filenames.gypi every javascript file that may go
into a browserify build, generate this list dynamically when the build files are
created by gyp.

tools/list-browserify-deps.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python
+import os
+import subprocess
+import sys
+
+
+SOURCE_ROOT = os.path.dirname(os.path.dirname(__file__))
+BROWSERIFY = os.path.join(SOURCE_ROOT, 'node_modules', '.bin', 'browserify')
+if sys.platform == 'win32':
+  BROWSERIFY += '.cmd'
+
+deps = subprocess.check_output([BROWSERIFY, '--list'] + sys.argv[1:])
+for dep in deps.split('\n'):
+    if dep:
+        dep = os.path.relpath(dep, SOURCE_ROOT)
+        if sys.platform == 'win32':
+            print('/'.join(dep.split('\\')))
+        else:
+            print(dep)