Implement flush and finish in RendererGL.
Implement flush and finish in RendererGL.

BUG=angleproject:883

Change-Id: Idc474c58d3971c5f7c2c2760cd21aefdf65424b4
Reviewed-on: https://chromium-review.googlesource.com/263502
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Kenneth Russell <1ebccd90c2bc76de65b6f830f2128a91ed69fdfb@chromium.org>
Tested-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

ecoal95/angle,larsbergstrom/angle,mybios/angle,csa7mdm/angle,mrobinson/rust-angle,vvuk/angle,ecoal95/angle,mikolalysenko/angle,domokit/waterfall,vvuk/angle,MSOpenTech/angle,ppy/angle,mrobinson/rust-angle,ghostoy/angle,larsbergstrom/angle,crezefire/angle,bsergean/angle,mikolalysenko/angle,mybios/angle,ghostoy/angle,csa7mdm/angle,crezefire/angle,ppy/angle,vvuk/angle,mybios/angle,csa7mdm/angle,ppy/angle,mrobinson/rust-angle,bsergean/angle,MSOpenTech/angle,mrobinson/rust-angle,ecoal95/angle,MSOpenTech/angle,ghostoy/angle,csa7mdm/angle,mlfarrell/angle,ecoal95/angle,domokit/waterfall,MSOpenTech/angle,mikolalysenko/angle,ecoal95/angle,larsbergstrom/angle,bsergean/angle,mlfarrell/angle,vvuk/angle,ghostoy/angle,mybios/angle,larsbergstrom/angle,ppy/angle,crezefire/angle,mrobinson/rust-angle,mlfarrell/angle,mikolalysenko/angle,mlfarrell/angle,bsergean/angle,crezefire/angle