Add SHARE_EXTENSION_ITEM_TYPE_COUNT to app_group::ShareExtensionItemType
Add SHARE_EXTENSION_ITEM_TYPE_COUNT to app_group::ShareExtensionItemType

Review URL: https://codereview.chromium.org/1810553002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381454}

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