Bug 1263197 - part 2 - remove temporary variable in nsPresContext's constructor; r=dholbert

In the upcoming patch to disable logging, we're going to conditionally
turn MOZ_LOG_TEST into a constant false.  But for this instance of
MOZ_LOG_TEST in nsPresContext, turning it into a constant means that
|log| becomes unused.  Since |log| isn't used after this point, let's
move the |gfxPlatform::GetLog| call into the MOZ_LOG_TEST call, removing
the temporary variable, and making the compiler happy.

layout/base/nsPresContext.cpp

@@ -261,8 +261,7 @@ nsPresContext::nsPresContext(nsIDocument* aDocument, nsPresContextType aType)
   mCounterStylesDirty = true;
 
   // if text perf logging enabled, init stats struct
-  LogModule* log = gfxPlatform::GetLog(eGfxLog_textperf);
-  if (MOZ_LOG_TEST(log, LogLevel::Warning)) {
+  if (MOZ_LOG_TEST(gfxPlatform::GetLog(eGfxLog_textperf), LogLevel::Warning)) {
     mTextPerf = new gfxTextPerfMetrics();
   }