[memory] Make sure size is properly aligned in FreePages.

- Adds a DCHECK to FreePages that size is a multiple of allocation
  granularity.
- Makes VirtualMemory::Free conform to this.

This is to conform more closely to Chromium's page allocator API.

Bug:chromium:756050

Change-Id: I673e1c225b8bd1009775de1597b575120bd06f8e
Reviewed-on: https://chromium-review.googlesource.com/898008Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51064}

src/allocation.cc

@@ -160,6 +160,7 @@ void* AllocatePages(void* address, size_t size, size_t alignment,
 }
 
 bool FreePages(void* address, const size_t size) {
+  DCHECK_EQ(0UL, size & (GetPageAllocator()->AllocatePageSize() - 1));
   bool result = GetPageAllocator()->FreePages(address, size);
 #if defined(LEAK_SANITIZER)
   if (result) {
@@ -260,7 +261,9 @@ void VirtualMemory::Free() {
   size_t size = size_;
   CHECK(InVM(address, size));
   Reset();
-  CHECK(FreePages(address, size));
+  // FreePages expects size to be aligned to allocation granularity. Trimming
+  // may leave size at only commit granularity. Align it here.
+  CHECK(FreePages(address, RoundUp(size, AllocatePageSize())));
 }
 
 void VirtualMemory::TakeControl(VirtualMemory* from) {

src/base/platform/platform-win32.cc

@@ -836,8 +836,7 @@ void* OS::Allocate(void* address, size_t size, size_t alignment,
 // static
 bool OS::Free(void* address, const size_t size) {
   DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % AllocatePageSize());
-  // TODO(bbudge) Add DCHECK_EQ(0, size % AllocatePageSize()) when callers
-  // pass the correct size on Windows.
+  DCHECK_EQ(0, size % AllocatePageSize());
   USE(size);
   return VirtualFree(address, 0, MEM_RELEASE) != 0;
 }