Add a mode which doesn't reset the mojo connection when the associated ExecutionContext is detached.

If the original code doesn't have `ContextDestroyed()`, then HeapMojoWrapperMode::kWithoutContextObserver should be used.

Bug: 1058076
Change-Id: I6ea02f1b663b968d70e7b3b475ce40f183592f57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2114020
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753122}