Fix ConvertPrecision.
Fix ConvertPrecision.

This method had never been fully implemented, and was returning
"highp" for every case value.

This should fix problems with the GPU FYI bots.

BUG=angle:691

Change-Id: Ie6ffa80cf2c063e506df06fb25d622227edeffa3
Reviewed-on: https://chromium-review.googlesource.com/206565
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

domokit/waterfall,vvuk/angle,xin3liang/platform_external_chromium_org_third_party_angle,MIPS/external-chromium_org-third_party-angle,jgcaaprom/android_external_chromium_org_third_party_angle,larsbergstrom/angle,mybios/angle,crezefire/angle,mrobinson/rust-angle,jgcaaprom/android_external_chromium_org_third_party_angle,ppy/angle,mlfarrell/angle,mybios/angle,mrobinson/rust-angle,bsergean/angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,bsergean/angle,mlfarrell/angle,jgcaaprom/android_external_chromium_org_third_party_angle,larsbergstrom/angle,MIPS/external-chromium_org-third_party-angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,ecoal95/angle,ecoal95/angle,nandhanurrevanth/angle,MSOpenTech/angle,mybios/angle,ppy/angle,mikolalysenko/angle,mikolalysenko/angle,xin3liang/platform_external_chromium_org_third_party_angle,android-ia/platform_external_chromium_org_third_party_angle,ecoal95/angle,ecoal95/angle,csa7mdm/angle,ppy/angle,csa7mdm/angle,android-ia/platform_external_chromium_org_third_party_angle,android-ia/platform_external_chromium_org_third_party_angle,mrobinson/rust-angle,MSOpenTech/angle,nandhanurrevanth/angle,ghostoy/angle,csa7mdm/angle,vvuk/angle,mikolalysenko/angle,android-ia/platform_external_chromium_org_third_party_angle,mlfarrell/angle,crezefire/angle,ghostoy/angle,domokit/waterfall,mrobinson/rust-angle,vvuk/angle,mikolalysenko/angle,nandhanurrevanth/angle,ghostoy/angle,bsergean/angle,MSOpenTech/angle,bsergean/angle,csa7mdm/angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,xin3liang/platform_external_chromium_org_third_party_angle,MIPS/external-chromium_org-third_party-angle,jgcaaprom/android_external_chromium_org_third_party_angle,MIPS/external-chromium_org-third_party-angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,mybios/angle,ghostoy/angle,MSOpenTech/angle,ecoal95/angle,nandhanurrevanth/angle,xin3liang/platform_external_chromium_org_third_party_angle,ppy/angle,crezefire/angle,larsbergstrom/angle,larsbergstrom/angle,mlfarrell/angle,crezefire/angle,mrobinson/rust-angle,vvuk/angle