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This checks subframes survives while being in the BackForwardCache. In
particular, what happens when loading a new main document in the same
process of the cached subframe? Does it survive? Currently: Yes.

+ Test: BackForwardCacheBrowserTest.SubframeSurviveCache1
+ Test: BackForwardCacheBrowserTest.SubframeSurviveCache2
+ Test: BackForwardCacheBrowserTest.SubframeSurviveCache3
+ Test: BackForwardCacheBrowserTest.SubframeSurviveCache4

Four new test cases:
 1) A1(B2) -> C3             -> A1(B2)
 2) A1(B2) -> B3             -> A1(B2)
 3) A1(B2) -> B3(A4)         -> A1(B2)   -> B3(A4)
 4) A1(B2) -> B3 -> A4 -> B5 -> A1(B2)

Bug: 511340
Change-Id: I25414030dc48d5bf7fcbe85b33dc00c9a2cd890d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627029
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666215}