There must be a matching Leave for each Enter. Otherwise it ends up
with a dead stack-allocated object in the timer chain.

Drive-by: There was also a bug in
RuntimeCallTimerScope::RuntimeCallTimerScope(HeapObject* ...) did create a
local object instead of calling an overloaded constructor.

BUG=chromium:669329

Change-Id: I9aa1c574a854af8beab3d8097efab3a726ad1c8d
Reviewed-on: https://chromium-review.googlesource.com/634511
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47613}