Remove <(webrtc_root) from source file entries.

This required to move the AGC tools source files
into webrtc/tools and create a new agc_test_utils target.

Since audio_codec_speed_tests.gypi referenced sources above,
the best approach I could come up with was to add an audio_coding.gypi
file at a higher level and move the targets in there (+ the includes from
modules.gyp which is an improvement IMO).

I also added a PRESUBMIT.py check to prevent new source
entries being added with <(webrtc_root) in the path.

BUG=4185
R=andrew@webrtc.org, tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37859004

Cr-Commit-Position: refs/heads/master@{#8197}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8197 4adac7df-926f-26a2-2b94-8c16560cd09d

PRESUBMIT.py

@@ -127,7 +127,7 @@ def _CheckNoSourcesAboveGyp(input_api, gyp_files, output_api):
   # Disallow referencing source files with paths above the GYP file location.
   source_pattern = input_api.re.compile(r'sources.*?\[(.*?)\]',
                                         re.MULTILINE | re.DOTALL)
-  file_pattern = input_api.re.compile(r"'(\.\./.*?)'")
+  file_pattern = input_api.re.compile(r"'((\.\./.*?)|(<\(webrtc_root\).*?))'")
   violating_gyp_files = set()
   violating_source_entries = []
   for gyp_file in gyp_files: