This is a reland of c0ed204d

Original change's description:
> Fix two issues with GLES2 devices
>
> * make sure maxColorAttachments is greater than 0
> * Do not sync read buffer if maxDrawBuffers is 0
>
> Bug: chromium:1167731
> Change-Id: Icd0fb5e65ec6ffdc0d6f6325ccd1207958922254
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2733268
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
> Commit-Queue: Peng Huang <penghuang@chromium.org>

Bug: chromium:1167731
Change-Id: Iccd7b0986c4728e362fccaeda6fbeff2860d888c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2743665Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>