IndexedDB: Replace IDL operation overloading with ScriptValue handling

The IDB spec has many IDL methods that take |any key| and in prose
have "if key is not a valid key or IDBKeyRange, throw; otherwise ...";
these ended up being implemented incrementally in Blink using
overloads in the IDL and .h/.cpp, and counting on nonstandard overload
handling in the code generator.

Update the Blink IDLs to mostly match the spec IDLs, and do the
ScriptValue to IDBKey or IDBKeyRange conversion via explicit calls to
a binding helper. This simplifies the IDL and implementations, and
will make future additions easier.

No behavior changes.

R=haraken@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22893058

git-svn-id: svn://svn.chromium.org/blink/trunk@156716 bbb929c8-8fbe-4397-9dbb-9b2b20218538