Fix crash in DOM snapshot agent

When taking as snapshot, the DOM snapshot agent holds a pointer to a
hashmap on the stack. The hashmap is destroyed when the devtools session
detaches, so if that happens while taking a snapshot we run into a
crash.

Fixed: 395032416
Change-Id: I4b784cb3105f6a760ca5b1d3dcbe1ea254b3ad3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6297444
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Philip Pfaffe <pfaffe@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425198}