[devtools] Allocate InspectorIssueStorage off-heap

The issue storage was changed to use the CDP structures instead of
Mojo structures. As such, the content of the storage is no longer
allocated on the oilpan heap so we can allocate the IssueStorage
class also off-heap.

R=caseq@chromium.org

Bug: chromium:1202622
Change-Id: If7a0ae607f258b2c0adbeafabbd7bbcf5ebc87ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2854787
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#879745}