Make targetDensity-dpi account for deviceScaleFactor correctly.

The fact that the deviceScaleFactor is applied unconditionally
needs to be accounted for in the targetDensity-dpi scale factor
computation.

BUG=None
TEST=WebFrameTest
R=abarth@chromium.org, aelias@chromium.org

Review URL: https://codereview.chromium.org/22574004

git-svn-id: svn://svn.chromium.org/blink/trunk@155837 bbb929c8-8fbe-4397-9dbb-9b2b20218538