Fix flaky Win compile

The Windows compile has been failing with the following flaky error:

5>Error: Failed to write to log file
"E:\b\build\slave\win-latest-dbg\build\src\build\Debug\obj\make_derived_sources\\make_derived_sources.log".
The process cannot access the file
'E:\b\build\slave\win-latest-dbg\build\src\build\Debug\obj\make_derived_sources\make_derived_sources.log'
because it is being used by another process.

This CL fixes the problem by giving the two "make_derived_sources" targets
different names so they won't fight over the same log file.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/30803002

git-svn-id: svn://svn.chromium.org/blink/trunk@160022 bbb929c8-8fbe-4397-9dbb-9b2b20218538