[ios] Change -sceneSessionID storage to std::string

Most of the code that use this property wants to convert it to an
std::string anyway, so store it as one to avoid expensive string
conversion at runtime.

Bug: none
Change-Id: I38e42737ec4b0c48dcf8afb450ca63959ed4e50a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6373164
Commit-Queue: Sylvain Defresne <[email protected]>
Reviewed-by: Mark Cogan <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1435663}
NOKEYCHECK=True
GitOrigin-RevId: 5a63a8d6e29aa2e19035a44f3db1e18177e8f1f6

• chrome/app/main_controller.mm[diff]
• chrome/app/profile/profile_controller.mm[diff]
• chrome/app/profile/search_engine_choice_profile_agent.mm[diff]
• chrome/browser/sessions/model/session_util.mm[diff]
• chrome/browser/shared/coordinator/scene/scene_state.h[diff]
• chrome/browser/shared/coordinator/scene/scene_state.mm[diff]
• chrome/browser/shared/coordinator/scene/scene_util.h[diff]
• chrome/browser/shared/coordinator/scene/scene_util.mm[diff]
• chrome/browser/shared/coordinator/scene/scene_util_unittest.mm[diff]