[OriginTrials] Raise an exception at compile time for OT-enabled methods

third_party/WebKit/Source/bindings/IDLExtendedAttributes.md

 # Blink IDL Extended Attributes
 
 [TOC]
 
 ## Introduction
 
 The main interest in extended attributes are their _semantics_: Blink implements many more extended attributes than the Web IDL standard, to specify various behavior.
 
 The authoritative list of allowed extended attributes and values is [bindings/IDLExtendedAttributes.txt](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/IDLExtendedAttributes.txt). This is complete but not necessarily precise (there may be unused extended attributes or values), since validation is run on build, but coverage isn't checked.
 
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     OriginTrialEnabled=MediaSession
 ] interface MediaSession { ... };
 ```
 
 When there is an active origin trial for the current execution context, the feature is enabled at runtime, and the binding would be exposed to the web. `[OriginTrialEnabled]` also includes a check for the associated runtime flag, so features can be enabled in that fashion, even without an origin trial.
 
 `[OriginTrialEnabled]` has similar semantics to `[RuntimeEnabled]`, and is intended as a drop-in replacement. For example, `[OriginTrialEnabled]` _cannot_ be applied to arguments, see `[RuntimeEnabled]` for reasoning. The key implementation difference is that `[OriginTrialEnabled]` wraps the generated code with `if (OriginTrials::FeatureNameEnabled(...)) { ...code... }`.
 
 For more information, see [RuntimeEnabledFeatures](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in) and [OriginTrialContext](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/origin_trials/OriginTrialContext.h).
 
+*** note
+**FIXME:** Currently, `[OriginTrialEnabled]` can only be applied to interfaces, attributes, and constants. Methods (including those generated by `iterable`, `setlike`, `maplike`, `serializer` and `stringifier`) are not supported. See [Bug 621641](https://crbug.com/621641).
+***
+
 ### [PostMessage] _(m)_
 
 Summary: Tells the code generator to generate postMessage method used in Workers, Service Workers etc.
 
 Usage: `[PostMessage]` can be specified on methods
 
 ```webidl
 [PostMessage] void postMessage(any message, optional sequence<Transferable> transfer);
 ```
 
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 ***