Angle: drop ANGLE_serialized_context_string from requestable This extension is always needed for replay, and it should always be enabled. By adding the extension using esOnlyExtension it is no longer marked as requestable and WebGL compatibility will no longer disable the extension. Bug: angleproject:5851 Change-Id: I11c8f684b3bfe76c219a9210df0a2c5cccb706f3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2853583 Commit-Queue: Gert Wollny <gert.wollny@collabora.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>