[bindings] Implement an ExperimentEnabled IDL extended attribute.

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

Index: third_party/WebKit/Source/core/experiments/Experiments.h
diff --git a/third_party/WebKit/Source/core/experiments/Experiments.h b/third_party/WebKit/Source/core/experiments/Experiments.h
index 868a6301cf52352c76602cd908c7cee1eeeae6d6..2d58029e342b70b3803815bfe508e838a3efa74b 100644
--- a/third_party/WebKit/Source/core/experiments/Experiments.h
+++ b/third_party/WebKit/Source/core/experiments/Experiments.h
@@ -25,6 +25,7 @@ namespace blink {
 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.
@@ -32,6 +33,8 @@ public:
 
 private:
     explicit Experiments();
+
+    static bool isApiEnabledInternal(ExecutionContext*, const String& apiName, String& errorMessage);
 };
 
 } // namespace blink