gn_helpers: Correctly handle comments followed by whitespace lines.

Without the fix, the modified unittest fails with:
"GNError: Expected an identifier"

With the fix, the unittest passes.

Bug: 1091082, 937821
Change-Id: I32591c64945dd918dfc341c8327fd530058b90ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2231286Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775299}

build/gn_helpers.py

@@ -284,6 +284,7 @@ class GNValueParser(object):
       val = self._ParseAllowTrailing()
       self.ConsumeWhitespace()
       self.ConsumeComment()
+      self.ConsumeWhitespace()
       d[ident] = val
 
     return d

build/gn_helpers_unittest.py

@@ -93,10 +93,15 @@ class UnitTest(unittest.TestCase):
     gn_args_lines = [
         '# Top-level comment.',
         'foo = true',
-        'bar = 1  # In-line comment.',
+        'bar = 1  # In-line comment followed by whitespace.',
+        ' ',
+        'baz = false',
     ]
-    self.assertEqual(gn_helpers.FromGNArgs('\n'.join(gn_args_lines)),
-                     {'foo': True, 'bar': 1})
+    self.assertEqual(gn_helpers.FromGNArgs('\n'.join(gn_args_lines)), {
+        'foo': True,
+        'bar': 1,
+        'baz': False
+    })
 
     # Lists should work.
     self.assertEqual(gn_helpers.FromGNArgs('foo=[1, 2, 3]'),