gpu: Clarify sized texture format is available only if ES3 context or immutable texture is supported

gpu/command_buffer/service/texture_manager.cc

Index: gpu/command_buffer/service/texture_manager.cc
diff --git a/gpu/command_buffer/service/texture_manager.cc b/gpu/command_buffer/service/texture_manager.cc
index dbe95db5b0987cc247aae5d060983263bef23dea..95d291ce21aaf9329e67192648d680fa7c29448c 100644
--- a/gpu/command_buffer/service/texture_manager.cc
+++ b/gpu/command_buffer/service/texture_manager.cc
@@ -231,6 +231,8 @@ class FormatTypeValidator {
         {GL_DEPTH24_STENCIL8, GL_DEPTH_STENCIL, GL_UNSIGNED_INT_24_8},
         {GL_DEPTH32F_STENCIL8, GL_DEPTH_STENCIL,
          GL_FLOAT_32_UNSIGNED_INT_24_8_REV},
+        // Exposed by GL_APPLE_texture_format_BGRA8888
+        {GL_BGRA8_EXT, GL_BGRA_EXT, GL_UNSIGNED_BYTE},
     };
 
     for (size_t ii = 0; ii < arraysize(kSupportedFormatTypes); ++ii) {