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This will allow using PreviousSessionInfo from
CrashKeyStringImpl::Set to set variations for
synthetic crash reports, see crrev.com/c/2462822

Notable changes:
 - moves PreviousSessionInfo from //ios/chrome to //components
 - moves IsMultiwindowSupported(), IsSceneStartupSupported() and generated
   multi_window_buildflags.h from //ios/chrome to //base/ios (so these
   functions could be used from components)
 - old IsMultiwindowSupported() and IsSceneStartupSupported() functions
   are kept in place to avoid changing the calling code and make
   cherry-picks easier. Old functions simply call new functions.

Bug: 1103752
Change-Id: I700b2c9811890c9ea03d6b1199ee797e2b3f0dcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462683
Commit-Queue: Eugene But <eugenebut@chromium.org>
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816818}