ui: Rename gfx::BufferUsage enum values and add BufferUsage::GPU_READ.

The problem with the current usage modes is that we currently use
SCANOUT for anything that doesn't require CPU access. When importing
a buffer we typically don't care about anything but having the GPU
be able to read from it. SCANOUT is not a good description of that
and currently means GPU read/write support.

This patch attempts to make usage more explicit and it solves the
use case where a buffer type can only provide GPU read and not GPU
write support (e.g. SHM fallback). Here are the new usage modes:

- GPU_READ
- GPU_READ_WRITE
- GPU_READ_CPU_READ_WRITE

GPU_READ is useful when importing a buffer and all you care about is
being able to use it for sampling and optionally scanout.

GPU_READ_WRITE is useful for things like WebGL or GPU raster where
we want the ability to write to the buffer using GL and potentially
use it for scanout.

GPU_READ_CPU_READ_WRITE is the usage mode that allows CPU access and
is used for one-copy and zero-copy texture initialization. There's
nothing preventing this mode from also supporting scanout.

- GPU_READ_CPU_READ_WRITE_PERSISTENT

This usage mode indicates that the data will be persistent across
Unmap()/Map() calls but will likely become part of the normal
GPU_READ_CPU_READ_WRITE usage once all buffer types support this.

BUG=538325
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1420923005

Cr-Commit-Position: refs/heads/master@{#356108}

1 files changed, 4 insertions, 3 deletions

diff --git a/cc/raster/one_copy_tile_task_worker_pool.cc b/cc/raster/one_copy_tile_task_worker_pool.cc
index 94c8a8e..18be9c2 100644
--- a/cc/raster/one_copy_tile_task_worker_pool.cc
+++ b/cc/raster/one_copy_tile_task_worker_pool.cc
@@ -388,14 +388,15 @@ void OneCopyTileTaskWorkerPool::PlaybackAndCopyOnWorkerThread(
     base::AutoUnlock unlock(lock_);
 
     // Allocate GpuMemoryBuffer if necessary. If using partial raster, we
-    // must allocate a buffer with BufferUsage PERSISTENT_MAP.
+    // must allocate a buffer with BufferUsage CPU_READ_WRITE_PERSISTENT.
     if (!staging_buffer->gpu_memory_buffer) {
       staging_buffer->gpu_memory_buffer =
           resource_provider_->gpu_memory_buffer_manager()
               ->AllocateGpuMemoryBuffer(
                   staging_buffer->size, BufferFormat(resource->format()),
-                  use_partial_raster_ ? gfx::BufferUsage::PERSISTENT_MAP
-                                      : gfx::BufferUsage::MAP);
+                  use_partial_raster_
+                      ? gfx::BufferUsage::GPU_READ_CPU_READ_WRITE_PERSISTENT
+                      : gfx::BufferUsage::GPU_READ_CPU_READ_WRITE);
       DCHECK_EQ(gfx::NumberOfPlanesForBufferFormat(
                     staging_buffer->gpu_memory_buffer->GetFormat()),
                 1u);