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This adds Get/GetOrCreate methods for ShareTargetInfo so we don't
accidentally create entries in the maps. Only a few places actually
need to create a new entry and this makes it easier to track those.

Bug: 1085067
Change-Id: I3d9f20a9a371a0af3968a485748d611840e299b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353334Reviewed-by: Himanshu Jaju <himanshujaju@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800125}