Bug 1173858 - Part 1: Add log module for layout. r=dholbert

Add a log module for use by layout. An analog to NS_WARN_IF_FALSE is provided
that has the same behavior as NS_WARN_IF_FALSE: it's debug only and emits a
message prefixed with '[pid] WARNING', includes the condition being checked,
file name and line number.

layout/base/LayoutLogging.cpp

@@ -0,0 +1,35 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Chromium headers must come before Mozilla headers.
+#include "base/process_util.h"
+
+#include "LayoutLogging.h"
+
+PRLogModuleInfo* GetLayoutLog()
+{
+  static PRLogModuleInfo* log = nullptr;
+  if (!log) {
+    log = PR_NewLogModule("layout");
+  }
+
+  return log;
+}
+
+namespace mozilla {
+namespace detail {
+
+void LayoutLogWarning(const char* aStr, const char* aExpr,
+                      const char* aFile, int32_t aLine)
+{
+  MOZ_LOG(GetLayoutLog(),
+          mozilla::LogLevel::Warning,
+          ("[%d] WARNING: %s: '%s', file %s, line %d",
+           base::GetCurrentProcId(),
+           aStr, aExpr, aFile, aLine));
+}
+
+} // namespace detail
+} // namespace mozilla

layout/base/LayoutLogging.h

@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef LayoutLogging_h
+#define LayoutLogging_h
+
+#include "mozilla/Logging.h"
+
+/**
+ * Retrieves the log module to use for layout logging.
+ */
+PRLogModuleInfo* GetLayoutLog();
+
+/**
+ * Use the layout log to warn if a given condition is false.
+ *
+ * This is only enabled in debug builds and the logging is only displayed if
+ * the environmental variable NSPR_LOG_MODULES includes "layout:2" (or higher). 
+ */
+#ifdef DEBUG
+#define LAYOUT_WARN_IF_FALSE(_cond, _msg)                                  \
+  PR_BEGIN_MACRO                                                           \
+    if (MOZ_LOG_TEST(GetLayoutLog(), mozilla::LogLevel::Warning) &&        \
+        !(_cond)) {                                                        \
+      mozilla::detail::LayoutLogWarning(_msg, #_cond, __FILE__, __LINE__); \
+    }                                                                      \
+  PR_END_MACRO
+#else
+#define LAYOUT_WARN_IF_FALSE(_cond, _msg) \
+  PR_BEGIN_MACRO                          \
+  PR_END_MACRO
+#endif
+
+namespace mozilla {
+namespace detail {
+
+void LayoutLogWarning(const char* aStr, const char* aExpr,
+                      const char* aFile, int32_t aLine);
+
+} // namespace detail
+} // namespace mozilla
+
+#endif // LayoutLogging_h

layout/base/moz.build

@@ -59,6 +59,7 @@ EXPORTS += [
     'FrameLayerBuilder.h',
     'FramePropertyTable.h',
     'LayerState.h',
+    'LayoutLogging.h',
     'nsArenaMemoryStats.h',
     'nsBidi.h',
     'nsBidiPresUtils.h',
@@ -109,6 +110,7 @@ UNIFIED_SOURCES += [
     'FrameLayerBuilder.cpp',
     'FramePropertyTable.cpp',
     'GeometryUtils.cpp',
+    'LayoutLogging.cpp',
     'MaskLayerImageCache.cpp',
     'nsBidi.cpp',
     'nsBidiPresUtils.cpp',