[headless] Force --screenshot command image size if --window-size

Old headless users commonly use --window-size switch to specify
screenshot size and this works because old headless has no chrome
around web contents and browser window size effectively defines
the resulting screenshot size.

With new headless this does not work because Chrome has tab strip
and omnibox at the top of the browser window, so screenshot vertical
size is typically smaller than expected.

This CL forces screenshot size to be the same as requested by
--window-size switch if it is present.

Bug: 328100308
Change-Id: If6b4f76ec6463af087db00b693d6afab4782149e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5341692
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Peter Kvitek <kvitekp@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1268207}