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This adds a helper, UpdateLabels(), to set all labels' visibility, text,
and style based on the current mode; this replaces the scattered pieces
throughout other functions.

The file name label's style changes based not on the mode but on the
enabled state of the whole view; update that in the functions that set
the view's enabled state.

The above two changes mean that UpdateColorsFromTheme() now only deals
with colors, and thus no longer need be called outside OnThemeChanged().

Renames |dangerous_download_label_| to the more accurate (and shorter!)
|warning_label_|.

For some reason UpdateMode() was also sometimes setting the file name
label's Y coordinate.  This is done in layout, and there's no reason to
do it here.

Bug: none
Change-Id: Ib7b9b77cc8ba11af609d9e25328f9207da66f6d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299141
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788914}