Fix partial-damage rasterization in SyncConfirmationUIDialogPixelTest

`sync-confirmation-app-refresh` uses
`shared_gradient_light_background.svg` as a full-viewport background
image (`<feGaussianBlur stdDeviation="66.5"/>`). During
`SyncConfirmationUIDialogPixelTest::ShowUi`, the dialog initializes and
displays in `ScreenMode.PENDING` (`#confirmButton` and `#notNowButton`
hidden with `.visibility-hidden`) before the asynchronous
`'screen-mode-changed'` WebUI event arrives, and `DisplayModal()`
focuses the WebContents so `#confirmButton[autofocus]` draws its 4px
focus ring before `TestBrowserUi::VerifyPixelUi` clears focus.

Because `VerifyPixelUi` only waits on the browser UI compositor and not
the renderer compositor, if either the button row visibility update or
focus ring removal is rasterized in a subsequent renderer frame after
an initial full frame, Skia clips `<feGaussianBlur>` evaluation to the
partial damage rect (`.action-row.dialog` or its union with
`#confirmButton`'s 4px outline), shifting background pixels inside that
rect by 1-2 RGB values and exceeding the fuzzy pixel diff threshold.

Clear focus on the dialog widget before waiting on the renderer, wait
for the WebUI element to exit `ScreenMode.PENDING`, and force a
full-viewport repaint before `ShowUi` returns.

Bug: 576742061
Change-Id: I1725ffef13985bcac0189e6c587eba0d5cb58e3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8640753
Auto-Submit: Russ Hamilton <behamilton@google.com>
Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1726170}