Require AllowShared with FlexibleArrayBufferView

All methods that have a parameter with the FlexibleArrayBufferView
extended attribute support AllowShared. In other words, it is possible
to use them with an ArrayBufferView backed by an ArrayBuffer or a
SharedArrayBuffer. This CL makes this an explicit requirement in the IDL
file.

Currently only WebGL APIs use the FlexibleArrayBufferView extended
attribute.

Bug: chromium:905449
Change-Id: I52f23d4ee07c169a525243f00dd8a96a58ce7075
Reviewed-on: https://chromium-review.googlesource.com/c/1345125
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611175}

third_party/blink/renderer/bindings/IDLExtendedAttributes.md

@@ -1433,6 +1433,8 @@ Summary: `[FlexibleArrayBufferView]` wraps a parameter that is known to be an Ar
 
 The FlexibleArrayBufferView itself can then either refer to an actual ArrayBufferView or a temporary copy (for small payloads) that may even live on the stack. The idea is that copying the payload on the stack and referring to the temporary copy saves creating global handles (resulting in weak roots) in V8. Note that `[FlexibleArrayBufferView]`  will actually result in a TypedFlexibleArrayBufferView wrapper for typed arrays.
 
+The FlexibleArrayBufferView extended attribute always requires the AllowShared extended attribute.
+
 Usage: Applies to arguments of methods. See modules/webgl/WebGLRenderingContextBase.idl for an example.
 
 ### [AllowShared] _(p)_

third_party/blink/renderer/bindings/scripts/v8_types.py

@@ -656,6 +656,8 @@ def v8_value_to_cpp_value(idl_type, extended_attributes, v8_value, variable_name
     if 'FlexibleArrayBufferView' in extended_attributes:
         if base_idl_type not in ARRAY_BUFFER_VIEW_AND_TYPED_ARRAY_TYPES:
             raise ValueError('Unrecognized base type for extended attribute "FlexibleArrayBufferView": %s' % (idl_type.base_type))
+        if 'AllowShared' not in extended_attributes:
+            raise ValueError('"FlexibleArrayBufferView" extended attribute requires "AllowShared" on %s' % (idl_type.base_type))
         base_idl_type = 'FlexibleArrayBufferView'
 
     if 'AllowShared' in extended_attributes and not idl_type.is_array_buffer_view_or_typed_array:

third_party/blink/renderer/bindings/tests/idls/core/test_object.idl

@@ -289,8 +289,8 @@ interface TestObject {
     void voidMethodArrayBufferArg(ArrayBuffer arrayBufferArg);
     void voidMethodArrayBufferOrNullArg(ArrayBuffer? arrayBufferArg);
     void voidMethodArrayBufferViewArg(ArrayBufferView arrayBufferViewArg);
-    void voidMethodFlexibleArrayBufferViewArg([FlexibleArrayBufferView] ArrayBufferView arrayBufferViewArg);
-    void voidMethodFlexibleArrayBufferViewTypedArg([FlexibleArrayBufferView] Float32Array typedArrayBufferViewArg);
+    void voidMethodFlexibleArrayBufferViewArg([FlexibleArrayBufferView, AllowShared] ArrayBufferView arrayBufferViewArg);
+    void voidMethodFlexibleArrayBufferViewTypedArg([FlexibleArrayBufferView, AllowShared] Float32Array typedArrayBufferViewArg);
     void voidMethodFloat32ArrayArg(Float32Array float32ArrayArg);
     void voidMethodInt32ArrayArg(Int32Array int32ArrayArg);
     void voidMethodUint8ArrayArg(Uint8Array uint8ArrayArg);