Allocator cleanup: remove reundant tc_set_new_mode in ProcessMemoryTest

Leftover from the previous cleanups.
Previously, when base did pretend to not know anything about allocator,
tc_set_new_mode was invoked by the content layer (in content_main).
But this required this base unittest to mimic the same content behavior.

These days, instead, tc_set_new_mode() is called by base's
EnableTerminationOnOutOfMemory(). This test is already invoking
EnableTerminationOnOutOfMemory() in its initialization, so there is
no need to perform a 2nd call to tc_set_new_mode.

Also this makes the code in this test conceptually cleaner and smaller.

BUG=564618
TEST=base_unittests

Review URL: https://codereview.chromium.org/1683153002

Cr-Commit-Position: refs/heads/master@{#374750}

base/process/memory_unittest.cc

@@ -148,12 +148,6 @@ TEST(ProcessMemoryTest, MacTerminateOnHeapCorruption) {
     !(defined(OS_WIN) && !defined(ALLOCATOR_SHIM)) && \
     !defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
 
-#if defined(USE_TCMALLOC)
-extern "C" {
-int tc_set_new_mode(int mode);
-}
-#endif  // defined(USE_TCMALLOC)
-
 namespace {
 const char *kOomRegex = "Out of memory";
 }  // namespace
@@ -171,12 +165,6 @@ class OutOfMemoryTest : public testing::Test {
     signed_test_size_(std::numeric_limits<ssize_t>::max()) {
   }
 
-#if defined(USE_TCMALLOC)
-  void SetUp() override { tc_set_new_mode(1); }
-
-  void TearDown() override { tc_set_new_mode(0); }
-#endif  // defined(USE_TCMALLOC)
-
  protected:
   void* value_;
   size_t test_size_;