Skip all config attributes that have the value of EGL_DONT_CARE.  From the
EGL 1.5 spec: "If EGL_DONT_CARE is specified as an attribute value, then
the attribute will not be checked.".

BUG=angleproject:1651

Change-Id: I30c95a1970543fb6f1d4b02d2babf3df61cad543
Reviewed-on: https://chromium-review.googlesource.com/533937Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>