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Since history always has normal browser state, even in incognito, pass down a
new load strategy forcing incognito from main_controller.
Catch a load in current tab for the other mode and forward to other service.
Fix crash with calling pre-render before checking if we have a web state.

Bug: 960728, 960732, 947005
Change-Id: I591e60ea7bf1e08ac27d6b63b8ba60fffa03e938
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599466
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658544}