Implement '[SecureContext]' IDL attribute for interfaces, methods and attributes.

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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 Whether `[Replaceable]` should be specified or not depends on the spec of each attribute.
 
 ### [SameObject] _(a)_
 
 Standard: [SameObject](http://heycam.github.io/webidl/#SameObject)
 
 Summary: Signals that a `readonly` attribute that returns an object type always returns the same object.
 
 This attribute has no effect on code generation and should simply be used in Blink IDL files if the specification uses it. If you want the binding layer to cache the resulting object, use `[SaveSameObject]`.
 
+### [SecureContext] _(a, i, m)_
+
+Standard: [SecureContext](https://heycam.github.io/webidl/#SecureContext)
+
+Summary: Interfaces and interface members with a `SecureContext` attribute are exposed only inside ["Secure Contexts"](https://w3c.github.io/webappsec-secure-contexts/).
+
 ### [TreatNullAs] _(a,p)_, [TreatUndefinedAs] _(a,p)_
 
 Standard: [TreatNullAs](http://heycam.github.io/webidl/#TreatNullAs)
 
 `[TreatUndefinedAs]` has been been removed from the spec.
 
 Summary: They control the behavior when a JavaScript null or undefined is passed to a DOMString attribute or parameter.
 
 Implementation: **Non-standard:** Web IDL specifies the EmptyString identifier for both these extended attributes, and `Null` and `Missing` for `TreatUndefinedAs`. Blink uses the `NullString` identifier instead of `EmptyString` which yields a Blink null string, corresponding to JavaScript `null`, for which both `String::IsEmpty()` and `String::IsNull()` return true, instead of the empty string `""`, which is empty but not `null`. This is for performance reasons; see [Strings in Blink](https://docs.google.com/a/google.com/document/d/1kOCUlJdh2WJMJGDf-WoEQhmnjKLaOYRbiHz5TiGJl14/edit) for reference. It also does not implement `TreatUndefinedAs=Null` or `TreatUndefinedAs=Missing`.
 
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