Prefer to use the OES_depth24 extension for WebGL's depth-only back buffers.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index df28ac3c8d30949935fafbe5b545df3f4a642160..4dea14b5ef679d0e13d11ebeea7c4e467eff90dc 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -104,21 +104,27 @@ PassRefPtr<DrawingBuffer> DrawingBuffer::create(PassOwnPtr<WebGraphicsContext3D>
     }
     ASSERT(extensionsUtil->supportsExtension("GL_OES_packed_depth_stencil"));
     extensionsUtil->ensureExtensionEnabled("GL_OES_packed_depth_stencil");
-    bool multisampleSupported = (extensionsUtil->supportsExtension("GL_CHROMIUM_framebuffer_multisample")
+
+    SupportedExtensions exts;
+    exts.multisample = (extensionsUtil->supportsExtension("GL_CHROMIUM_framebuffer_multisample")
         || extensionsUtil->supportsExtension("GL_EXT_multisampled_render_to_texture"))
         && extensionsUtil->supportsExtension("GL_OES_rgb8_rgba8");
-    if (multisampleSupported) {
+    if (exts.multisample) {
         extensionsUtil->ensureExtensionEnabled("GL_OES_rgb8_rgba8");
         if (extensionsUtil->supportsExtension("GL_CHROMIUM_framebuffer_multisample"))
             extensionsUtil->ensureExtensionEnabled("GL_CHROMIUM_framebuffer_multisample");
         else
             extensionsUtil->ensureExtensionEnabled("GL_EXT_multisampled_render_to_texture");
     }
-    bool discardFramebufferSupported = extensionsUtil->supportsExtension("GL_EXT_discard_framebuffer");
-    if (discardFramebufferSupported)
+    exts.packedDepthStencil = true;
+    exts.depth24 = extensionsUtil->supportsExtension("GL_OES_depth24");
+    if (exts.depth24)
+        extensionsUtil->ensureExtensionEnabled("GL_OES_depth24");
+    exts.discardFramebuffer = extensionsUtil->supportsExtension("GL_EXT_discard_framebuffer");
+    if (exts.discardFramebuffer)
         extensionsUtil->ensureExtensionEnabled("GL_EXT_discard_framebuffer");
 
-    RefPtr<DrawingBuffer> drawingBuffer = adoptRef(new DrawingBuffer(std::move(context), extensionsUtil.release(), multisampleSupported, true, discardFramebufferSupported, preserve, requestedAttributes));
+    RefPtr<DrawingBuffer> drawingBuffer = adoptRef(new DrawingBuffer(std::move(context), extensionsUtil.release(), exts, preserve, requestedAttributes));
     if (!drawingBuffer->initialize(size)) {
         drawingBuffer->beginDestruction();
         return PassRefPtr<DrawingBuffer>();
@@ -131,11 +137,12 @@ void DrawingBuffer::forceNextDrawingBufferCreationToFail()
     shouldFailDrawingBufferCreationForTesting = true;
 }
 
+DrawingBuffer::SupportedExtensions::SupportedExtensions() :
+    multisample(false), packedDepthStencil(false), depth24(false), discardFramebuffer(false) {}
+
 DrawingBuffer::DrawingBuffer(PassOwnPtr<WebGraphicsContext3D> context,
     PassOwnPtr<Extensions3DUtil> extensionsUtil,
-    bool multisampleExtensionSupported,
-    bool packedDepthStencilExtensionSupported,
-    bool discardFramebufferSupported,
+    const SupportedExtensions& supportedExtensions,
     PreserveDrawingBuffer preserve,
     WebGraphicsContext3D::Attributes requestedAttributes)
     : m_preserveDrawingBuffer(preserve)
@@ -148,9 +155,10 @@ DrawingBuffer::DrawingBuffer(PassOwnPtr<WebGraphicsContext3D> context,
     , m_extensionsUtil(std::move(extensionsUtil))
     , m_size(-1, -1)
     , m_requestedAttributes(requestedAttributes)
-    , m_multisampleExtensionSupported(multisampleExtensionSupported)
-    , m_packedDepthStencilExtensionSupported(packedDepthStencilExtensionSupported)
-    , m_discardFramebufferSupported(discardFramebufferSupported)
+    , m_multisampleExtensionSupported(supportedExtensions.multisample)
+    , m_packedDepthStencilExtensionSupported(supportedExtensions.packedDepthStencil)
+    , m_depth24ExtensionSupported(supportedExtensions.depth24)
+    , m_discardFramebufferSupported(supportedExtensions.discardFramebuffer)
     , m_fbo(0)
     , m_depthStencilBuffer(0)
     , m_depthBuffer(0)
@@ -697,7 +705,18 @@ void DrawingBuffer::resizeDepthStencil(const IntSize& size)
     if (!m_requestedAttributes.depth && !m_requestedAttributes.stencil)
         return;
 
-    if (m_packedDepthStencilExtensionSupported) {
+    if (m_requestedAttributes.depth && !m_requestedAttributes.stencil && m_depth24ExtensionSupported) {
+        if (!m_depthBuffer)
+            m_depthBuffer = m_context->createRenderbuffer();
+        m_context->bindRenderbuffer(GL_RENDERBUFFER, m_depthBuffer);
+        if (m_antiAliasingMode == MSAAImplicitResolve)
+            m_context->renderbufferStorageMultisampleEXT(GL_RENDERBUFFER, m_sampleCount, GL_DEPTH_COMPONENT24, size.width(), size.height());
+        else if (m_antiAliasingMode == MSAAExplicitResolve)
+            m_context->renderbufferStorageMultisampleCHROMIUM(GL_RENDERBUFFER, m_sampleCount, GL_DEPTH_COMPONENT24, size.width(), size.height());
+        else
+            m_context->renderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, size.width(), size.height());
+        m_context->framebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, m_depthBuffer);
+    } else if (m_packedDepthStencilExtensionSupported) {
         if (!m_depthStencilBuffer)
             m_depthStencilBuffer = m_context->createRenderbuffer();
         m_context->bindRenderbuffer(GL_RENDERBUFFER, m_depthStencilBuffer);