[heap-profiler] Fix reporting of fake global objects.

The global objects lookup code mistakingly reports weakly referenced
JSGlobalObject's as normal one. It should not.

The fix just adds is_weak check into V8HeapExplorer::SetGcSubrootReference
the rest is formatting.

Bug: chromium:747382
Change-Id: I3fc62317dd3d8728d261f27bd58654aff13eb6fe
Reviewed-on: https://chromium-review.googlesource.com/585385Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46913}