build_utils.WriteDepfile: Add an assert for passing a string as |inputs|

I've made this mistake a couple of times :(
Without a check, it writes single-letter files to the depfile.

Change-Id: I0cf315573d2ed92b6b23fcf095e7396980d8d03d
Reviewed-on: https://chromium-review.googlesource.com/c/1355324Reviewed-by: Peter Wen <wnwen@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612262}

build/android/gyp/util/build_utils.py

@@ -527,6 +527,7 @@ def AddDepfileOption(parser):
 
 def WriteDepfile(depfile_path, first_gn_output, inputs=None, add_pydeps=True):
   assert depfile_path != first_gn_output  # http://crbug.com/646165
+  assert not isinstance(inputs, basestring)  # Easy mistake to make
   inputs = inputs or []
   if add_pydeps:
     inputs = _ComputePythonDependencies() + inputs