Request exact GL context versions from ANGLE.

ANGLE is moving to support the more standard behaviour of returning a context
version that is backwards compatible with the requested version but this new
behaviour conflicts with what Chrome expects with the passthrough command
decoder.

BUG=angleproject:3425

Change-Id: I107b194fe2e3ad8e1707ab55155d6d74ee0b4735
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602813Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658577}

ui/gl/gl_context_egl.cc

@@ -50,6 +50,11 @@
 #define EGL_ROBUST_RESOURCE_INITIALIZATION_ANGLE 0x3453
 #endif /* EGL_ANGLE_display_robust_resource_initialization */
 
+#ifndef EGL_ANGLE_create_context_backwards_compatible
+#define EGL_ANGLE_create_context_backwards_compatible 1
+#define EGL_CONTEXT_OPENGL_BACKWARDS_COMPATIBLE_ANGLE 0x3483
+#endif /* EGL_ANGLE_create_context_backwards_compatible */
+
 #ifndef EGL_CONTEXT_PRIORITY_LEVEL_IMG
 #define EGL_CONTEXT_PRIORITY_LEVEL_IMG 0x3100
 #define EGL_CONTEXT_PRIORITY_HIGH_IMG 0x3101
@@ -183,6 +188,14 @@ bool GLContextEGL::Initialize(GLSurface* compatible_surface,
     DCHECK(!attribs.robust_resource_initialization);
   }
 
+  if (GLSurfaceEGL::HasEGLExtension(
+          "EGL_ANGLE_create_context_backwards_compatible")) {
+    // Request a specific context version. The Passthrough command decoder
+    // relies on the returned context being the exact version it requested.
+    context_attributes.push_back(EGL_CONTEXT_OPENGL_BACKWARDS_COMPATIBLE_ANGLE);
+    context_attributes.push_back(EGL_FALSE);
+  }
+
   // Append final EGL_NONE to signal the context attributes are finished
   context_attributes.push_back(EGL_NONE);
   context_attributes.push_back(EGL_NONE);