[composebox] Adds CaptureScreenshot to TabContextualizationCtrl API

As part of displaying viewport screenshot in the tab selector, this CL:

- Adds CaptureScreenshot to TabContextualizationController public API.
  The screenshot can be optionally downscaled to optimize for display.
- Moves image encoding and downscaling from //components/omnibox/ to
  //components/lens to reuse in TabContextualizationController.
- Adds browser tests to verify screenshot capture and downscaling.
- Replaces unsafe usage of base::Unretained(this) with WeakPtr.
- Replaces SkBitmap::empty() checks with SkBitmap::drawsNothing()
  which which better accounts for various empty states.

Bug: 443939809
Change-Id: I25f71696e5de9e23a5a8cfbb9bcd72b05a13c6cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6928916
Reviewed-by: Christian Xu <christianxu@chromium.org>
Reviewed-by: Jason Hu <hujasonx@google.com>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1513141}