webview render: Update a parameter name that was for "render context".
webview render: Update a parameter name that was for "render context".

Apparently didn't rename the parameter for this function when renaming
the type.  Update it to match usage elsewhere.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile