Attempt to get SharedSamplerTest.PhysicalMemory passing with libc++.

Currently fails on the libc++ bot with

../../chrome/browser/task_manager/sampling/shared_sampler_win_unittest.cc(144):
    error: Expected: (physical_bytes()) >= (initial_value + allocated_size),
             actual: 15224832 vs 19419136

The reason seems to be that memory use is only counted if the memory is
referenced, and with libc++ the compiler is apparently able to optimize away
the memory's initialization.  Add some reading code to make sure the
memory references remain.

Bug: 801780
Change-Id: I8edef2eda4be8b4344ebc934768ecd58ee39496b
Reviewed-on: https://chromium-review.googlesource.com/956571
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542225}

chrome/browser/task_manager/sampling/shared_sampler_win_unittest.cc

@@ -7,6 +7,7 @@
 #include <windows.h>
 
 #include <memory>
+#include <numeric>
 
 #include "base/callback.h"
 #include "base/macros.h"
@@ -133,10 +134,17 @@ TEST_F(SharedSamplerTest, PhysicalMemory) {
 
   int64_t initial_value = physical_bytes();
 
-  // Allocate a large continuos block of memory.
+  // Allocate a large continuous block of memory.
   const int allocated_size = 4 * 1024 * 1024;
   std::vector<uint8_t> memory_block(allocated_size);
 
+  // It appears the allocation is not counted when allocated, but when actually
+  // accessed. vector's constructor does access the memory to initialize it,
+  // but if the memory then isn't read the compiler might optimize away the
+  // initialization. So read the memory to prevent that.
+  uint8_t sum = std::accumulate(memory_block.begin(), memory_block.end(), 0);
+  EXPECT_EQ(0, sum);
+
   StartRefresh(REFRESH_TYPE_PHYSICAL_MEMORY);
   WaitUntilRefreshDone();