subpixels: layout.css.devPixelsPerPx should be integer

Reason Fraction devPixelsPerPx have some render bug Fraction devPixelsPerPx make the screen size have deviation, increase the perception

Example 1.5 devPixelsPerPx https://user-images.githubusercontent.com/15178410/30520588-2e3d3b1e-9be3-11e7-889a-9f7fc5608e8a.png

When Desktop Environment is fraction DPI, the TOR browser auto fraction devPixelsPerPx, cause the problem

Trac:
Username: zcyzcy88888@126.com

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