Before this commit, methods in the chrome.devtools.inspectedWindow
namespace and the Resource object (such as eval, getResources,
getContent, and setContent) only supported callback-based asynchronous
patterns.

After this commit, the inspectedWindow namespace and Resource object
methods have been promisified. They now return a Promise if no callback
is provided, while maintaining full backwards compatibility with
callback callers.

Bug: 496604552, 496604551, 496616808
Change-Id: I5cefc3a0a5fcf7fa1d80d9b3cd996e01a2097796
TAG=agy
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7802274
Commit-Queue: Philip Pfaffe <pfaffe@chromium.org>
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Auto-Submit: Justin Lulejian <jlulejian@chromium.org>