BrowserSettings: update desktop UA string to match Chrome 30

Now that we're using a Chromium-based WebView which closely matches
Chrome 30, we should report as a newer Chrome version when masquerading
as a desktop browser.

Change-Id: I301a8cfac9a83e66a6749872943e80ff5c4740a1
Replace removed NARROW_COLUMNS layout mode with TEXT_AUTOSIZING

For reasons which I completely fail to understand, Google elected not to
implement the very useful NARROW_COLUMNS layout mode (double-tap to
reflow text) in Chromium WebView, and instead chose to implement
TEXT_AUTOSIZING (buggy heuristic-based font inflation).  But complaining
isn't going to get it back, and font inflation is better than nothing on
small screens, so replace NARROW_COLUMNS with TEXT_AUTOSIZING to restore
some support for resizing text to be legible on screen.

Change-Id: Ieeb03d38e8a7386e8e8f811c570aa17bfabf4337