Revert "[Partition Alloc] Don't redundantly call madvise redundantly."

This reverts commit be7b68a7.

Reason for revert: Lost the crucial CHECK(!ret) in a merge conflict, and re-adding it causes macOS to fail (EINVAL, invalid address). That requires further investigation.

Original change's description:
> [Partition Alloc] Don't redundantly call madvise redundantly.
> 
> When
> 
>   #ifndef MADV_FREE
>   #define MADV_FREE MADV_DONTNEED
>   #endif
> 
> we would call madvise(..., MADV_FREE); and then, if that failed, call
> madvise(..., MADV_DONTNEED). This would result in pointlessly calling
> madvise(..., MADV_DONTNEED) and then trying again instead of CHECKing
> immediately.
> 
> This is more of a readability refactor than a performance-relevant change,
> obviously.
> 
> Thanks to ajwong for noticing this!
> 
> BUG=766882,755284
> 
> Change-Id: If8cbe14f38dfe2bd126bc24f79e670c45b29c8d5
> Reviewed-on: https://chromium-review.googlesource.com/754038
> Reviewed-by: Primiano Tucci <primiano@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Commit-Queue: Chris Palmer <palmer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#514635}

TBR=ajwong@chromium.org,palmer@chromium.org,primiano@chromium.org,haraken@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 766882, 755284
Change-Id: I2b71c2f6773c38109d17c151e5a81aa50bc46b10
Reviewed-on: https://chromium-review.googlesource.com/759058Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515091}

base/allocator/partition_allocator/page_allocator.cc

@@ -24,6 +24,10 @@
 #include <errno.h>
 #include <sys/mman.h>
 
+#ifndef MADV_FREE
+#define MADV_FREE MADV_DONTNEED
+#endif
+
 #ifndef MAP_ANONYMOUS
 #define MAP_ANONYMOUS MAP_ANON
 #endif
@@ -288,7 +292,6 @@ bool SetSystemPagesAccess(void* address,
 
 void DecommitSystemPages(void* address, size_t length) {
   DCHECK_EQ(0UL, length & kSystemPageOffsetMask);
-
 #if defined(OS_POSIX)
   // In POSIX, there is no decommit concept. Discarding is an effective way of
   // implementing the Windows semantics where the OS is allowed to not swap the
@@ -321,27 +324,22 @@ bool RecommitSystemPages(void* address,
 
 void DiscardSystemPages(void* address, size_t length) {
   DCHECK_EQ(0UL, length & kSystemPageOffsetMask);
-
 #if defined(OS_POSIX)
-  int ret = -1;
 #if defined(OS_MACOSX)
   // On macOS, MADV_FREE_REUSABLE has comparable behavior to MADV_FREE, but also
   // marks the pages with the reusable bit, which allows both Activity Monitor
   // and memory-infra to correctly track the pages.
-  ret = madvise(address, length, MADV_FREE_REUSABLE);
+  int ret = madvise(address, length, MADV_FREE_REUSABLE);
 #else
-#if defined(MADV_FREE)
-  ret = madvise(address, length, MADV_FREE);
+  int ret = madvise(address, length, MADV_FREE);
+#endif
   if (ret != 0 && errno == EINVAL) {
-    // MADV_FREE only works on Linux 4.5+. If the request failed, retry with the
-    // older MADV_DONTNEED. (Note that MADV_FREE being defined at compile time
-    // doesn't imply runtime support.)
+    // MADV_FREE only works on Linux 4.5+ . If request failed,
+    // retry with older MADV_DONTNEED . Note that MADV_FREE
+    // being defined at compile time doesn't imply runtime support.
     ret = madvise(address, length, MADV_DONTNEED);
   }
-#else
-  ret = madvise(address, length, MADV_DONTNEED);
-#endif  // MADV_FREE
-#endif  // OS_MACOSX
+  CHECK(!ret);
 #else
   // On Windows discarded pages are not returned to the system immediately and
   // not guaranteed to be zeroed when returned to the application.
@@ -365,7 +363,7 @@ void DiscardSystemPages(void* address, size_t length) {
     void* ret = VirtualAlloc(address, length, MEM_RESET, PAGE_READWRITE);
     CHECK(ret);
   }
-#endif  // OS_POSIX
+#endif
 }
 
 bool ReserveAddressSpace(size_t size) {