Land Khronos's latest egl.h and eglplatform.h in third_party/khronos.

Also use them for the EGLImageWrappingTests, rather than ANGLE's copy.

Eventually, we may want to replace these with egl.xml and a
parser/generator, but for now landing the headers is the most
expedient path.

Bug: dawn:810
Change-Id: I2068349eaef5a93813f76c2881a2dac6aad73670
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94562
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn/tests/BUILD.gn

@@ -597,9 +597,9 @@ source_set("white_box_tests_sources") {
     sources += [ "white_box/MetalAutoreleasePoolTests.mm" ]
   }
 
-  if (dawn_enable_opengles && defined(dawn_angle_dir)) {
+  if (dawn_enable_opengles) {
     sources += [ "white_box/EGLImageWrappingTests.cpp" ]
-    include_dirs = [ "${dawn_angle_dir}/include" ]
+    include_dirs = [ "//third_party/khronos" ]
   }
 
   libs = []