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Automatically generated ARCMigrate commit
Notable issues:
* Replaced 2 instances of replaceOldTabModel:(TabModel**)oldTabModel withTabModel:(TabModel*)newTabModel method with an equivalent construct that does not pass pointers to pointers to objc objects. Feel free to suggest a better way that I can implement in a follow-up CL.
* some weak pointer dereferencing replaced with strong pointer dereferencing in blocks
* In tab_model_snapshot.mm, added a bridge cast. I'm not sure if this should be a conversion to std::string, because std::stringstream.operator<<(NSString*) can be both.

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2810193002
Cr-Commit-Position: refs/heads/master@{#468652}