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First create an EGLImage from a given gl texture.
Then export the EGLImage as dmabuf.

In addition to the new unit test included in this CL,
this will be useful for the subsequent CL,
https://chromium-review.googlesource.com/c/chromium/src/+/766787
which will enable existing VaapiDrmPicture when passing --use-gl=egl.
While existing VaapiTFPPicture(X11) will be left for --use-gl=desktop.

Added unit test ui/gl/gl_image_native_pixmap_unittest.cc
The test is enabled on non-ozone but should work on ozone too.
If any of the pre-conditions fail then the test is skipped.
Specifically it requires the extension EGL_MESA_image_dma_buf_export.
So it works on Intel, AMDGPU and nouveau drivers but skipped on NVIDIA.

Bug: 785201
Test: gl_unittests --gtest_filter=*NativePixmap*

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Id9b4e56aaaaf5fd9994f4b144ce74a234d848253
Reviewed-on: https://chromium-review.googlesource.com/766687
Commit-Queue: Julien Isorce <julien.isorce@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522052}