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This patch adds the following dumps in v8 memory dump provider:
- v8/isolate/contexts/detached_context
- v8/isolate/contexts/native_context
Both dumps contain an "object_count" scalar value.

The value of detached_context is the number of contexts that were
detached (due to iframe navigation or removal) and not yet garbage
collected. This number being non-zero indicates potential memory leak.

The value of native_context is the number of the top-level contexts
created for the main window and iframes. Increase of this number
over time indicates a memory leak.

Bug: 793789
Change-Id: I4753454747c398fac4fc153fcccb1288d28f2a2d
Reviewed-on: https://chromium-review.googlesource.com/822336
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524438}