Use `libjpeg_gyp_path' instead of depending on libjpeg_turbo.

Make the behavior consistent with the rest of the gyp files in Blink and do
not force the dependency on libjpeg_turbo: instead, use the
`libjpeg_gyp_path' variable, which is appropriately set when, for example,
the user passes `use_system_libjpeg' to the gyp.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/19284003

git-svn-id: svn://svn.chromium.org/blink/trunk@154280 bbb929c8-8fbe-4397-9dbb-9b2b20218538