Fix a typo so we get the correct CHROMIUM_SRC_DIR and add it to
INCLUDEPATH so that QtCreator indexes Chromium headers.
Also add NINJAFLAGS environment variable to ninja command line
to be able to specify options to ninja, like how many jobs it
should run when using icecc.

Change-Id: Ib494dc5c71d4e3a4eac419118c9cf6b1c474b6da
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>