Fix base_unittests compilation failure on official build bots.

On some official build bots, r531606 causes a -Wunused-result error.

BUG=805615

Change-Id: I0f4f51a6eb07d67f3b4c552af7d5f115b5852095
Reviewed-on: https://chromium-review.googlesource.com/884505
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531707}

base/files/dir_reader_posix_unittest.cc

@@ -33,8 +33,10 @@ TEST(DirReaderPosixUnittest, Read) {
   ASSERT_TRUE(dir);
 
   char wdbuf[PATH_MAX];
-  getcwd(wdbuf, PATH_MAX);
+  if (getcwd(wdbuf, PATH_MAX))
     LOG(ERROR) << "Current directory: " << wdbuf;
+  else
+    PLOG(ERROR) << "getcwd() failed with ";
   const int prev_wd = open(".", O_RDONLY | O_DIRECTORY);
   PLOG_IF(ERROR, prev_wd < 0) << "opening . failed with ";
   DCHECK_GE(prev_wd, 0);