Implement GLHelperReadbackSupport for GLHelper usage.

content/common/gpu/client/gl_helper.h

Index: content/common/gpu/client/gl_helper.h
diff --git a/content/common/gpu/client/gl_helper.h b/content/common/gpu/client/gl_helper.h
index 46db753207217ae13659920803008f68b739df8c..daf8ad93327fc5a1ab3e0e9ffccf032fbddaee3d 100644
--- a/content/common/gpu/client/gl_helper.h
+++ b/content/common/gpu/client/gl_helper.h
@@ -169,6 +169,12 @@ class CONTENT_EXPORT GLHelper {
     SCALER_QUALITY_BEST = 3,
   };
 
+  enum FormatSupport {
+    FORMAT_NOT_SUPPORTED = 0,
+    FORMAT_SUPPORTED,
+    FORMAT_NONE,

[no sievers, 2014/02/19 19:09:39]

nit: FORMAT_NONE -> FORMAT_SUPPORT_UNKNOWN

[sivag, 2014/02/21 11:40:51]

Done.

+  };
+
   // Copies the block of pixels specified with |src_subrect| from |src_texture|,
   // scales it to |dst_size|, and writes it into |out|.
   // |src_size| is the size of |src_texture|. The result is of format GL_BGRA
@@ -277,8 +283,9 @@ class CONTENT_EXPORT GLHelper {
   // size of the framebuffer.
   void CopyTextureFullImage(GLuint texture, const gfx::Size& size);
 
-  // Check whether rgb565 readback is supported or not.
-  bool CanUseRgb565Readback();
+  // Checks whether the readback using passed texture format
+  // is supported or not.

[no sievers, 2014/02/19 19:09:39]

nit: I think the comment could still be a bit clearer so that one does not
incorrectly assume that 'IsReadbackConfigSupported(565) == true' means that you
can read a RGBA8 texture back into a 565 bitmap using ReadbackTexture(A)Sync()
in this class.

[sivag, 2014/02/21 11:40:51]

Done.

+  bool IsReadBackConfigSupported(const SkBitmap::Config& bitmap_format);
 
   // A scaler will cache all intermediate textures and programs
   // needed to scale from a specified size to a destination size.
@@ -328,6 +335,10 @@ class CONTENT_EXPORT GLHelper {
   // 0 if GL_EXT_draw_buffers is not available.
   GLint MaxDrawBuffers();
 
+ private:
+  // Helper functions for checking the supported texture formats.
+  bool SupportsFormat(GLint format, GLint type);
+
  protected:
   class CopyTextureToImpl;
 
@@ -340,8 +351,7 @@ class CONTENT_EXPORT GLHelper {
   gpu::ContextSupport* context_support_;
   scoped_ptr<CopyTextureToImpl> copy_texture_to_impl_;
   scoped_ptr<GLHelperScaling> scaler_impl_;
-  bool initialized_565_format_check_;
-  bool support_565_format_;
+  FormatSupport format_support_table_[SkBitmap::kConfigCount];
 
   DISALLOW_COPY_AND_ASSIGN(GLHelper);
 };