[Extensions] Add `scale` parameter to `ImageDetails`.

Add `scale` parameter to `ImageDetails` to enable custom scaling during screen capture.

This parameter aligns with Mozilla's implementation of `ImageDetails`
(https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/extensionTypes/ImageDetails#scale).
and allows for overriding the default value which can be useful in cases
where the user would like a specific resolution of the capture image.

Bug: 423658618
Change-Id: Ib9da4c9cc29a41d39cc46245ed1618290ce2c952
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6734895
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Tim <tjudkins@chromium.org>
Commit-Queue: Eva Su <evasu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1488559}