Revert "PageAllocator: Fix DecommitSystemPages() semantics"

This reverts commit 8a24491093779658b7392f2718e691203bd45fcb.

Caused a 5.4-6.6% DOM rendering performance regression in mac in
dromaeo.domcoremodify and a 15.8% memory usage regression for android
in system_health.memory_mobile.

The perf regression might be explainable by the tightening of the
page protections, but the memory regression doesn't make sense.

Reverting to give time to investigate.

TBR=palmer,haraken

Bug: 775949, 776755
Change-Id: I7475fe2e384f9846f64da276ee0754ede1b009e9
Reviewed-on: https://chromium-review.googlesource.com/740091
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511967}

• base/allocator/partition_allocator/page_allocator.cc[diff]
• base/allocator/partition_allocator/page_allocator.h[diff]
• third_party/WebKit/Source/platform/heap/HeapPage.cpp[diff]
• third_party/WebKit/Source/platform/heap/PageMemory.cpp[diff]