Fix a crash in GLSurfaceEglReadback::SwapBuffers()

The crash is because GLSurfaceEglReadback::SwapBuffers() uses
glReadPixels() to read back pixels from framebuffer to a buffer.
It assumes the current GL_PIXEL_PACK_BUFFER_BINDING is 0, so
glReadPixels() will store pixels in the buffer passed in instead
of buffer binded at GL_PIXEL_PACK_BUFFER. However the
GL_PIXEL_PACK_BUFFER_BINDING is not 0 sometimes. Fix the problem
by binding 0 to GL_PIXEL_PACK_BUFFER before glReadPixels(), and
restore it after glTreadPixels(). This change also sets 0 to
GL_READ_FRAMEBUFFER_BINDING to make sure reading from the right
surface.

Bug: 1073882
Change-Id: Ic4bf35fb02baccb87664b7488af06d77d5197446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163080
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Auto-Submit: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762161}

ui/ozone/common/gl_surface_egl_readback.cc

@@ -44,9 +44,29 @@ bool GLSurfaceEglReadback::IsOffscreen() {
 gfx::SwapResult GLSurfaceEglReadback::SwapBuffers(
     PresentationCallback callback) {
   const gfx::Size size = GetSize();
+
+  GLint read_fbo = 0;
+  GLint pixel_pack_buffer = 0;
+  glGetIntegerv(GL_READ_FRAMEBUFFER_BINDING, &read_fbo);
+  glGetIntegerv(GL_PIXEL_PACK_BUFFER_BINDING, &pixel_pack_buffer);
+
+  // Make sure pixels are read from fbo 0.
+  if (read_fbo)
+    glBindFramebufferEXT(GL_READ_FRAMEBUFFER, 0);
+
+  // Make sure pixels are stored into |pixels_| instead of buffer binding to
+  // GL_PIXEL_PACK_BUFFER.
+  if (pixel_pack_buffer)
+    glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
+
   glReadPixels(0, 0, size.width(), size.height(), GL_BGRA, GL_UNSIGNED_BYTE,
                pixels_.get());
 
+  if (read_fbo)
+    glBindFramebufferEXT(GL_READ_FRAMEBUFFER, read_fbo);
+  if (pixel_pack_buffer)
+    glBindBuffer(GL_PIXEL_PACK_BUFFER, pixel_pack_buffer);
+
   gfx::SwapResult swap_result = gfx::SwapResult::SWAP_FAILED;
   gfx::PresentationFeedback feedback;