Send notifications when exiting memory pressure (/net/disk_cache)

The current convention is to not send notification when the system exit
memory pressure, this isn't ideal as it doesn't let stateful listeners
know that they should stop their intervention.
This CL was uploaded by git cl split.

R=morlovich@chromium.org

Change-Id: Ief0ffac5125e55a233e483e2a749e65927fc6894
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017046
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735919}

net/disk_cache/memory/mem_backend_impl.cc

@@ -364,8 +364,6 @@ void MemBackendImpl::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
   switch (memory_pressure_level) {
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE:
-      // Not supposed to get this here, but if there is no problem, there is
-      // no problem...
       break;
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE:
       EvictTill(max_size_ / 2);