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This CL fixes all the MetricsWebContentsObserverTest which
checks for ERR_NO_IPCS_RECEIVED event during deletion of page after
navigating away. As with BFCache, page is kept alive after navigating.

ERR_NO_IPCS_RECEIVED is logged during desturction of a page
which happens when page navigates away. But with BackForwardCache,
page is stored in back-forward cache.

We also add BackForwardCache::IsBackForwardCacheFeatureEnabled()
to //content/public to check if BackForwardCache is enabled or not.

Bug: 1157426
Change-Id: I989a033f55a7f626b426c3c013c136f00f35e704
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611007Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Sreeja Kamishetty <sreejakshetty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840969}