Adding PRESUBMIT check to avoid mixing C, C++ and Objc-C/Obj-C++.

The error message will be something like:

GN targets cannot mix .c (or .cc) and .m (or .mm) source files.
Please create a separate target for each collection of sources.
Mixed sources:
{
  BUILD_GN_PATH: [
    [
      TARGET_NAME,
      [
        SOURCES
      ]
    ],
    ...
  ],
  ...
}

Bug: webrtc:7743
Change-Id: I45dd2c621b830e5aeb081fa8d17c9497a49c2554
Reviewed-on: https://webrtc-review.googlesource.com/1980
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19897}

presubmit_test_mocks.py

@@ -25,6 +25,15 @@ class MockInputApi(object):
     # pylint: disable=unused-argument
     return self.files
 
+  def ReadFile(self, affected_file, mode='rU'):
+    filename = affected_file.AbsoluteLocalPath()
+    for f in self.files:
+      if f.LocalPath() == filename:
+        with open(filename, mode) as f:
+          return f.read()
+    # Otherwise, file is not in our mock API.
+    raise IOError, "No such file or directory: '%s'" % filename
+
 
 class MockOutputApi(object):
   """Mock class for the OutputApi class.
@@ -71,3 +80,6 @@ class MockFile(object):
 
   def LocalPath(self):
     return self._local_path
+
+  def AbsoluteLocalPath(self):
+    return self._local_path