Setting EGL_WIDTH or EGL_HEIGHT results in mFixedWidth and mFixedHeight
changing their values.

Whenever the swap chain is tested for needing a resize, surfaces with
mFixedSize are treated as if they should be resized to mFixedWidth and
mFixedHeight from their current size. This way fixed-size surfaces
can be resized - it's enough to do a couple of eglSurfaceAttrib() calls
and then either let ANGLE check surface for needing a resize in due course,
or force that by calling eglWaitNative().

BUG=angleproject:2725

Change-Id: I2932d8ecd5a2f10500e4aa8583a169aa09bf01bb
Reviewed-on: https://chromium-review.googlesource.com/1139055
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>