Add ANGLE workaround to prefer flushes over VRAM usage

On the whole, https://codereview.chromium.org/1286203002/ (Defer flushes if kPreferNoIO is specified) improved performance but it did cause a performance regression on ANGLE. This CL disables the deferral of flushes on ANGLE until we can add a separate incremental flushing mechanism.

TBR=bsalomon@google.com

BUG=skia:4201
BUG=521529

Review URL: https://codereview.chromium.org/1287193008
6 files changed