Don't hardcode gn target path for licence generation.

This fixes a bug where the generated license for the framework was missing all third party dependencies.

Bug: None
Change-Id: I81331f7f4d32e3302ce6ce0430272904820ce6d6
Reviewed-on: https://chromium-review.googlesource.com/530689
Commit-Queue: Kári Tristan Helgason <kthelgason@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Reviewed-by: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18556}

tools_webrtc/ios/generate_licenses.py

@@ -63,6 +63,7 @@ class LicenseBuilder(object):
     # The `sum` function flattens the 2d list.
     third_party_libs = sum([GetThirdPartyLibraries(buildfile, self.target_name)
                             for buildfile in self.buildfile_dirs], [])
+    assert len(third_party_libs) > 0
 
     # Generate amalgamated list of libraries. Will exit with error if a
     # lib is unrecognized.