WebView may change surfaces due to resizing. So we need to support
switching surfaces with the external context.

Bug: angleproject:5509
Change-Id: Id91eed092a63b3740fd796e0a3cb819ae18baaa9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2639077Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>