Add sleep option to web_to_(skp|mskp) shell scripts.

On slower machines, invoking |chrome.gpuBenchmarking.printToSkPicture|
too early after the headless browser is launched leads to no .skp
files being captured.  Adding a sleep after the launch of the headless
browser rectifies the issue.

Bug: NONE
Change-Id: I39178f8336b794a6f293bd337aca0b6f85a07360
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297805
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed