[bindings] Implement an ExperimentEnabled IDL extended attribute.

third_party/WebKit/Source/core/experiments/Experiments.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef Experiments_h
 #define Experiments_h
 
 #include "core/CoreExport.h"
 #include "core/dom/DOMException.h"
 #include "core/dom/ExecutionContext.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
 // The Experimental Framework (EF) provides limited access to experimental APIs,
 // on a per-origin basis.  This class provides an interface to API implementers,
 // for checking if the experimental API should be enabled for the current
 // context.
 //
 // Experimental APIs are defined by string names, provided by the implementers.
 // The EF code does not maintain an enum or constant list for experiment names.
 // Instead, the EF validates the name provided by the API implementation against
 // any provided API keys.
 // TODO(chasej): Link to documentation, or provide more detail on keys, .etc
 class CORE_EXPORT Experiments {
 public:
     static bool isApiEnabled(ExecutionContext*, const String& apiName, String& errorMessage);
+    static bool isApiEnabled(ExecutionContext*, const String& apiName);

[chasej, 2015/12/22 16:58:26]

Makes sense to add this overload without an error message.  The addition of an
errorMessage parameter was based on ExecutionContext.isSecureContext(), which
provides an overload without the parameter.

Note that crrev.com/1538663003/ has landed, which added code generation to
provide explicit ExperimentalFeatures::xxxEnabled() checks for every experiment.
 The bindings code should use those generated methods, rather than calling
isApiEnabled() directly. Additionally, the generated "xxxEnabled()" method will
first check if the corresponding runtime enabled feature.  I believe this could
simplify the logic for the bindings code:
- If [ExperimentEnabled] and [RuntimeEnabled] attributes are found, for the
*same* name, the [RuntimeEnabled] attribute can be ignored.  The
[ExperimentEnabled] attribute can take precedence.

The generated methods do not currently provide an overload without an error
message.  I'll add that in a separate CL.

This also raises the point of naming and potential confusion between
[RuntimeEnabled] and [ExperimentEnabled]. That's a larger issue that goes beyond
the bindings code.  We'll need to address separately, so I'm going to start a
thread for discussion.

     // Creates a NotSupportedError exception with a message explaining to
     // external developers why the API is disabled and how to join API
     // experiments.
     static DOMException* createApiDisabledException(const String& apiName);
 
 private:
     explicit Experiments();
+
+    static bool isApiEnabledInternal(ExecutionContext*, const String& apiName, String& errorMessage);
 };
 
 } // namespace blink
 
 #endif // Experiments_h