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When users pinch in Reader Mode, the page would zoom in or out as if it
is a normal web page allowing user-zoom. At the end of pinch gesture, the
page would do text reflow. These pinch-to-zoom and text reflow effects
are not native, but are emulated using CSS and JavaScript.

In order to achieve near-native zooming and panning frame rate, fake 3D
transform is used so that the layer doesn't repaint for each frame.

After the text reflow, the web content shown in the viewport should
roughly be the same paragraph before zooming.

The control point of font size is the html element, so that both "em" and
"rem" are adjusted. Accordingly, font size of body is no longer specified
in CSS in unit of pixel.

Some CSS styles and animations are updated to fix issues specific to
resizing.

BUG=445632

Review URL: https://codereview.chromium.org/1009703002

Cr-Commit-Position: refs/heads/master@{#326945}