Turn down OOM memory test allocation a wee bit

The OOM death tests used an allocation value of MAX_LONG minus 12k.
The problem with that value is that, on systems like the Mac DTKs
that have 16k VM pages, that gets rounded up and wraps. Therefore,
abstract it into “subtract three pages”.

Bug: 1121749
Change-Id: I6c519200cbd848c095b741d78f51317ea6ef18b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376887
Commit-Queue: Avi Drissman <avi@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801831}

base/process/memory_unittest.cc

@@ -17,6 +17,7 @@
 #include "base/compiler_specific.h"
 #include "base/debug/alias.h"
 #include "base/memory/aligned_memory.h"
+#include "base/process/process_metrics.h"
 #include "base/strings/stringprintf.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -129,9 +130,10 @@ class OutOfMemoryTest : public testing::Test {
  public:
   OutOfMemoryTest()
       : value_(nullptr),
-        // Make test size as large as possible minus a few pages so
-        // that alignment or other rounding doesn't make it wrap.
-        test_size_(std::numeric_limits<std::size_t>::max() - 12 * 1024),
+        // Make test size as large as possible minus a few pages so that
+        // alignment or other rounding doesn't make it wrap.
+        test_size_(std::numeric_limits<std::size_t>::max() -
+                   3 * base::GetPageSize()),
         // A test size that is > 2Gb and will cause the allocators to reject
         // the allocation due to security restrictions. See crbug.com/169327.
         insecure_test_size_(std::numeric_limits<int>::max()),