In "Implement EGL_ANGLE_explicit_context", backslashes were introduced in
a file path, which broke generation on non-Windows machines. This fixes
the issue.

Bug:angleproject:1395
Change-Id: I188c42c2a92afec37d7cb7dac5bffd7e855a0d2b
Reviewed-on: https://chromium-review.googlesource.com/1072509Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>