gpu: Retry to allocate an image when SCANOUT fails

CreateSharedImage with usage SHARED_IMAGE_USAGE_SCANOUT tries to allocate
the underlying buffer with BufferUsage::SCANOUT.

BufferUsage::SCANOUT might fail in some circumentances when a buffer can
still be allocated, but it's not possible to use it for scanout (e.g: on
virtio_gpu AddFramebuffer will fail for buffers smaller than a certain
threshold).

This changes the logic of CreateSharedImage so that in cases where we fail
to allocate a buffer for scanout, we try again to allocate just for GL.

Bug: 1024019
Test: crrev.com/c/1918316 in webgl_conformance_tests on chromeos-amd64-generic-rel
Change-Id: I20615e0780e47425a4074204f706bc66018e2071
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1933198
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718970}

gpu/command_buffer/service/shared_image_backing_factory_gl_texture.cc

@@ -883,9 +883,18 @@ SharedImageBackingFactoryGLTexture::CreateSharedImage(
     image = image_factory_->CreateAnonymousImage(
         size, format_info.buffer_format, gfx::BufferUsage::SCANOUT,
         &is_cleared);
-    // A SCANOUT image should not require copy.
-    DCHECK(!image || image->ShouldBindOrCopy() == gl::GLImage::BIND);
-    if (!image || !image->BindTexImage(target)) {
+    // Scanout images have different constraints than GL images and might fail
+    // to allocate even if GL images can be created.
+    if (!image) {
+      // TODO(dcastagna): Use BufferUsage::GPU_READ_WRITE instead
+      // BufferUsage::GPU_READ once we add it.
+      image = image_factory_->CreateAnonymousImage(
+          size, format_info.buffer_format, gfx::BufferUsage::GPU_READ,
+          &is_cleared);
+    }
+    // The allocated image should not require copy.
+    if (!image || image->ShouldBindOrCopy() != gl::GLImage::BIND ||
+        !image->BindTexImage(target)) {
       LOG(ERROR) << "CreateSharedImage: Failed to "
                  << (image ? "bind" : "create") << " image";
       api->glDeleteTexturesFn(1, &service_id);