Use GLHelper support to validate format before processing texture.

content/common/gpu/client/gl_helper_readback_support.cc

Index: content/common/gpu/client/gl_helper_readback_support.cc
diff --git a/content/common/gpu/client/gl_helper_readback_support.cc b/content/common/gpu/client/gl_helper_readback_support.cc
index e773ca90782a6dc684bec38c7afb455e1cfe925c..36fc825a79067fbd288b81c9c0fec8adec0f577d 100644
--- a/content/common/gpu/client/gl_helper_readback_support.cc
+++ b/content/common/gpu/client/gl_helper_readback_support.cc
@@ -25,8 +25,8 @@ void GLHelperReadbackSupport::InitializeReadbackSupport() {
   for (int i = 0; i <= kLastEnum_SkColorType; ++i) {
     format_support_table_[i] = GLHelperReadbackSupport::NOT_SUPPORTED;
   }
-  // TODO(sikugu): kAlpha_8_SkColorType support check is failing on mesa.
-  // See crbug.com/415667.
+
+  CheckForReadbackSupport(kAlpha_8_SkColorType);
   CheckForReadbackSupport(kRGB_565_SkColorType);
   CheckForReadbackSupport(kARGB_4444_SkColorType);
   CheckForReadbackSupport(kRGBA_8888_SkColorType);
@@ -38,6 +38,9 @@ void GLHelperReadbackSupport::CheckForReadbackSupport(
     SkColorType texture_format) {
   bool supports_format = false;
   switch (texture_format) {
+    case kAlpha_8_SkColorType:
+      supports_format = SupportsFormat(GL_ALPHA, GL_UNSIGNED_BYTE);
+      break;
     case kRGB_565_SkColorType:
       supports_format = SupportsFormat(GL_RGB, GL_UNSIGNED_SHORT_5_6_5);
       break;
@@ -135,6 +138,15 @@ GLHelperReadbackSupport::GetReadbackConfig(SkColorType color_type,
   *type = GL_UNSIGNED_BYTE;
   GLenum new_format = 0, new_type = 0;
   switch (color_type) {
+    case kAlpha_8_SkColorType:
+      if (format_support_table_[color_type] ==
+          GLHelperReadbackSupport::SUPPORTED) {
+        *format = GL_ALPHA;
+        *type = GL_UNSIGNED_BYTE;
+        *bytes_per_pixel = 1;
+        return GLHelperReadbackSupport::SUPPORTED;
+      }
+      break;
     case kRGB_565_SkColorType:
       if (format_support_table_[color_type] ==
           GLHelperReadbackSupport::SUPPORTED) {