Modify ChromeExtensionsCapabilityTest.testCanInspectBackgroundPage
to stop using obsolete LaunchApp API.

Bug: chromedriver:2278
Change-Id: Ided03a33e59ee2251e47550f9b6ae0d8b96f3f19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053172
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741545}