Transmit browser zoom level and css zoom factor separately.

After these recent changes:

https://chromium-review.googlesource.com/c/chromium/src/+/5341652
https://chromium-review.googlesource.com/c/chromium/src/+/5571113

... VisualProperties.zoom_level is a composite of zoom level set by the
user via the browser UI and any CSS zoom factor applied to an <iframe>
in its embedding document. However, there is now a use case (PEPC) which
needs to distinguish between those two kinds of zoom.

This patch splits apart browser zoom level and CSS zoom factor and
transmits them separately. There should be no functional change; a
follow-up patch will access WebFrameWidgetImpl::css_zoom_factor_ from
PEPC code.

One effect of this change is that VisualProperties.zoom_level is now the
same for all widgets belonging to a WebContents, so we no longer need
RenderWidgetHostView::GetZoomLevel() or AdjustedChildZoom(), which was
an override mechanism for webview API added by the above-linked CL.

Bug: chromium:352046941

Change-Id: I2d50d3684f07d9eb8958110505552afa1590f413
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5719026
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1341340}