Print more debug info on semaphore failure.

BUG=chromium:605349
LOG=NO

Review URL: https://codereview.chromium.org/1921213004

Cr-Commit-Position: refs/heads/master@{#35825}

src/base/platform/semaphore.cc

@@ -101,7 +101,9 @@ Semaphore::~Semaphore() {
 
 void Semaphore::Signal() {
   int result = sem_post(&native_handle_);
-  CHECK_EQ(0, result);
+  if (result != 0) {
+    V8_Fatal(__FILE__, __LINE__, "Semaphore signal failure: %d\n", errno);
+  }
 }