Make gypi_to_gn skip target_defaults sections.

Add 'target_defaults' to the list of top level keys ignored by gypi_to_gn.py
so it can process files that contain that.

BUG=795329

Change-Id: I3733a7efb97366827febf87ec1be95ad8723be23
Reviewed-on: https://chromium-review.googlesource.com/830114Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524449}

build/gypi_to_gn.py

@@ -117,6 +117,7 @@ def LoadPythonDictionary(path):
   elements_to_strip = [
     'conditions',
     'target_conditions',
+    'target_defaults',
     'targets',
     'includes',
     'actions',