Im struggling with this as well, help would be greatly appreciated.
ran gclient sync (ok, except the last one: "running '/usr/bin/python src/build/gyp_chromium'")
cd'ed into src/
did the GYP_GENERATORS='ninja'
ran ./build/gyp_chromium content/content.gyp --no-circular-check
all good so far.
then trying $ninja -C out/Release nw -j4
throws the error: $ninja: fatal: chdir to 'out/Release' - No such file or directory
I then attempted to $mkdir out/Release
but then $ninja -C out/Release nw -j4
throws $ninja: error: loading 'build.ninja': No such file or directory
help!