Move the map logic to the buffer storage implementation.
Move the map logic to the buffer storage implementation.

Refactoring patch only.

BUG=angle:511

Change-Id: Id69efe15ecb32d711a79cc5ff612853e29377368
Reviewed-on: https://chromium-review.googlesource.com/193242
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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