Consolidate GPU buffer implementations

Consolidates all the different buffer implementations into a single
GrBuffer class. This will allow us to add new buffer types, use DSA in
OpenGL, track buffer bindings by unique ID, cache buffers without
respect to the type of data they have been used for previously, etc.
This change is strictly a refactor; it introduces no change in
functionality.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1825393002

Committed: https://skia.googlesource.com/skia/+/8b1bff29675afd25843439eade634a57f68fe16f

Review URL: https://codereview.chromium.org/1825393002
diff --git a/src/gpu/GrBatchFlushState.cpp b/src/gpu/GrBatchFlushState.cpp
index 52261a1..f01d888 100644
--- a/src/gpu/GrBatchFlushState.cpp
+++ b/src/gpu/GrBatchFlushState.cpp
@@ -20,11 +20,11 @@
     , fLastFlushedToken(0) {}
 
 void* GrBatchFlushState::makeVertexSpace(size_t vertexSize, int vertexCount,
-                                         const GrVertexBuffer** buffer, int* startVertex) {
+                                         const GrBuffer** buffer, int* startVertex) {
     return fVertexPool.makeSpace(vertexSize, vertexCount, buffer, startVertex);
 }
 
 uint16_t* GrBatchFlushState::makeIndexSpace(int indexCount,
-                                            const GrIndexBuffer** buffer, int* startIndex) {
+                                            const GrBuffer** buffer, int* startIndex) {
     return reinterpret_cast<uint16_t*>(fIndexPool.makeSpace(indexCount, buffer, startIndex));
 }