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Previously, we were directly generating header files for
OpenGLES2 interface. This is not aligned with other Pepper
APIs. The header file generated from the generated IDL file
should be compatible with the previous one. To keep OpenGLES2
interfaces in struct namespace, this change introduces
"force_struct_namespace" attribute to the generator script.

TEST=trybots
BUG=411799

Review URL: https://codereview.chromium.org/538903002

Cr-Commit-Position: refs/heads/master@{#293904}