DevTools: Prevent clear-storage-a11y-test from timing out

Refactored the code to use common methods (which didn't
exist when the test was written), and fix the time out
in the test.

The fix for the time out was to reduce the size of the
allocation we were making as data into indexeddb.

Change-Id: Id5889c923e421dc195a5a9edf58fedd37e39ca9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1927219
Commit-Queue: Mike Jackson <mjackson@microsoft.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718490}
3 files changed