Use GL_RGBA renderbuffer for antialiased, no-alpha DrawingBuffers.

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 84315810ffd7db1393fb9fcedfd524e2f09c5f46..6193831fb5f242c93be21038711895ee7ab155f4 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -339,8 +339,9 @@ DrawingBuffer::TextureParameters DrawingBuffer::chromiumImageTextureParameters()
         parameters.creationInternalColorFormat = GL_RGB;
         parameters.internalColorFormat = GL_RGBA;
     } else {
-        parameters.creationInternalColorFormat = GL_RGB;
-        parameters.internalColorFormat = GL_RGB;
+        GLenum format = defaultBufferRequiresAlphaChannelToBePreserved() ? GL_RGBA : GL_RGB;
+        parameters.creationInternalColorFormat = format;
+        parameters.internalColorFormat = format;
     }
 
     // Unused when CHROMIUM_image is being used.
@@ -360,9 +361,10 @@ DrawingBuffer::TextureParameters DrawingBuffer::defaultTextureParameters()
         parameters.creationInternalColorFormat = GL_RGBA;
         parameters.colorFormat = GL_RGBA;
     } else {
-        parameters.internalColorFormat = GL_RGB;
-        parameters.creationInternalColorFormat = GL_RGB;
-        parameters.colorFormat = GL_RGB;
+        GLenum format = defaultBufferRequiresAlphaChannelToBePreserved() ? GL_RGBA : GL_RGB;
+        parameters.creationInternalColorFormat = format;
+        parameters.internalColorFormat = format;
+        parameters.colorFormat = format;
     }
     return parameters;
 }
@@ -1051,6 +1053,8 @@ GLenum DrawingBuffer::getMultisampledRenderbufferFormat()
         return GL_RGBA8_OES;
     if (m_colorBuffer.imageId && contextProvider()->getCapabilities().chromium_image_rgb_emulation)
         return GL_RGBA8_OES;
+    if (contextProvider()->getCapabilities().disable_webgl_rgb_multisampling_usage)

[Ken Russell (switch to Gerrit), 2016/05/19 00:06:41]

It seems a little strange that this checks a different condition than the other
code (i.e., defaultBufferRequiresAlphaChannelToBePreserved). Are you sure all of
the possible legal combinations work?

[erikchen, 2016/06/02 00:14:06]

defaultBufferRequiresAlphaChannelToBePreserved calls this method, which is
expected and correct, now that I removed the fallback code. 

[Ken Russell (switch to Gerrit), 2016/06/02 00:46:10]

Ah, I see. Thanks for the explanation.

+        return GL_RGBA8_OES;
     return GL_RGB8_OES;
 }