Add dpi support for WebView.

Add dpi support for WebView.

In the "viewport" meta tag, you can specify "target-densityDpi".
If it is not specified, it uses the default, 160dpi as of today.
Then the 1.0 scale factor specified in the viewport tag means 100%
on G1 and 150% on Sholes. If you set "target-densityDpi" to
"device-dpi", then the 1.0 scale factor means 100% on both G1 and Sholes.

Implemented Safari's window.devicePixelRatio and css media query
device-pixel-ratio.

So if you use "device-dpi" and modify the css for font-size and image
src depending on window.devicePixelRatio, you can get a better page on
Sholes/Passion.

Here is a list of options for "target-densityDpi".

device-dpi:    Use the device's native dpi as target dpi.
low-dpi:       120dpi
medium-dpi:    160dpi, which is also the default as of today
high-dpi:      240dpi
<number>:      We take any number between 70 and 400 as a valid target dpi.

Fix http://b/issue?id=2071943