Restore ScopedLogger and add documentation.

third_party/WebKit/Source/wtf/ScopedLogger.md

+# Debugging with ScopedLogger
+
+## Overview
+
+ScopedLogger is a logger that shows nested calls by indenting.
+
+For example, if you were debugging a layout issue you could add a ScopedLogger
+to the top of the `LayoutBlock::layout` function:
+
+```c++
+void LayoutBlock::layout()
+{
+    WTF_CREATE_SCOPED_LOGGER(logger, "layout %s", debugName().utf8().data());
+    ...
+```
+
+The arguments of the `WTF_CREATE_SCOPED_LOGGER` macro are the name of the
+object, followed by the first log message, followed by printf-style varargs.
+In the above example, the log message includes the debug name of the block that
+is currently being laid out.
+
+ScopedLogger wraps log messages in parentheses, with indentation proportional to
+the number of instances.  This makes it easy to see the flow of control in the
+output, particularly when instrumenting recursive functions.  Here is some of
+the output of the above example when laying out www.google.com:
+
+```
+( layout LayoutView #document
+  ( layout LayoutBlockFlow HTML
+    ( layout LayoutBlockFlow BODY id='gsr' class='hp vasq'
+      ( layout LayoutBlockFlow (relative positioned) DIV id='viewport' class='ctr-p'
+        ( layout LayoutBlockFlow DIV id='doc-info' )
+        ( layout LayoutBlockFlow DIV id='cst' )
+        ( layout LayoutBlockFlow (positioned) A )
+        ( layout LayoutBlockFlow (positioned) DIV id='searchform' class='jhp' )
+      )
+    )
+  )
+)
+```
+
+## Appending to a ScopedLogger
+
+Every ScopedLogger has an initial log message, which is often sufficient.  But
+you can also write additional messages to an existing ScopedLogger with
+`WTF_APPEND_SCOPED_LOGGER`.  For example:
+
+```c++
+    // further down in LayoutBlock::layout...
+
+    if (needsScrollAnchoring) {
+        WTF_APPEND_SCOPED_LOGGER(logger, "restoring scroll anchor");
+        getScrollableArea()->scrollAnchor().restore();
+    }
+```
+
+## Conditional ScopedLoggers
+
+It's often useful to create a ScopedLogger only if some condition is met.
+Unfortunately, the following doesn't work correctly:
+
+```c++
+void foo() {
+    if (condition) {
+        WTF_CREATE_SCOPED_LOGGER(logger, "foo, with condition");
+        // Oops: logger exits scope prematurely!
+    }
+    bar();  // any ScopedLogger in bar won't nest
+}
+```
+
+To guard a ScopedLogger construction with a condition without restricting its
+scope, use `WTF_CREATE_SCOPED_LOGGER_IF`:
+
+```c++
+void foo() {
+    WTF_CREATE_SCOPED_LOGGER_IF(logger, condition, "message");
+    bar();
+}
+```
+
+## Requirements
+
+The ScopedLogger class and associated macros are defined in
+[Assertions.h](Assertions.h), which most Blink source files already include
+indirectly.  ScopedLogger can't be used outside of Blink code yet.
+
+The ScopedLogger macros work in debug builds by default.  They are compiled out
+of release builds, unless your `GYP_DEFINES` or GN args file includes one of the
+following:
+
+* `dcheck_always_on`: enables assertions and logging
+* `blink_logging_always_on`: enables logging, but not assertions
+
+(These are the same rules that govern `WTF_LOG` and other WTF logging macros.)
+
+The macro names are cumbersome to type, but most editors can be configured to
+make this easier.  For example, you can add the following to a Sublime Text key
+binding file to make Ctrl+Alt+L insert a ScopedLogger:
+
+```
+  { "keys": ["ctrl+alt+l"], "command": "insert",
+    "args": {"characters": "WTF_CREATE_SCOPED_LOGGER(logger, \"msg\");"}
+  }
+```