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Moved the key for this pref and the logic for getting / setting it from
ScriptingPermissionsModifier to ExtensionPrefs. This lays the groundwork
for making it cleaner to set this value on installation if permissions
were withheld on installation.

Bug: 984069
Change-Id: I5580dfbdd57f4e2a3e92dd7b62b02bf04b35fee9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762720Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Tim Judkins <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689170}