[devtools] Use protocol format for issue storage instead of Mojo

This CL changes the "InspectorIssueStorage" to use the protocol
format. In the future, this will have the advantage of getting rid
of the conversion step from Mojo to Protocol, if issues are filed
directly in the protocol format.

R=sigurds@chromium.org

Bug: chromium:1202622
Change-Id: I7a6a781611cfadcf8750f43abaacee0820face8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2850694


Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#876964}