IndexedDB: Remove artificial IDBAny.idl

An "IDBAny" typed union is used to represent variant return value
types in many places in modules/indexeddb, and allow lazy conversion
from WebCore types to v8 types (e.g. strings, wrapped objects, etc).
This was done using an artificial IDBAny.idl that doesn't correspond
to any spec, in order for other types to be able to specify return
values of type IDBAny and have the code generator call toV8(IDBAny*)
to do the conversion.

Over time, many of the call sites have been changed to return
ScriptValue and do explicit conversion instead, which lets the Blink
IDL use "any" like the Web IDL. This patch goes the rest of the way
and removes the custom binding and IDL for IDBAny and makes
all conversions explicit.

R=haraken
BUG=225873

Review URL: https://codereview.chromium.org/99013005

git-svn-id: svn://svn.chromium.org/blink/trunk@163051 bbb929c8-8fbe-4397-9dbb-9b2b20218538