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Mime handlers create a nested WebContents to contain the mime handler
extension. Currently, these run with a separate session storage
namespace, resulting in session storage being cleared if a tab recreates
a mime handler, such as due to a reload. Change mime handler WebContents
creation to inherit the session storage namespaces of their embedders so
the session storage behaves similarly to iframes within a single tab.

Bug: 819761
Change-Id: I3b4f653836190ed86572eb12cdbeef3fbc157683
Reviewed-on: https://chromium-review.googlesource.com/1116633
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570814}