Reland with a Windows build fix.

We want to move the Chromium initialization of the ANGLE platform from
being Windows-specific to being EGL-specific. Because the mechanisms by
which the ANGLE library is loaded are OS-specific, it is inconvenient to
use dlsym and friends to get the ANGLEPlatform entry points. Instead
we expose the through eglGetProcAddress which is readily available in the
EGL-specific part of Chromium.

BUG=angleproject:1621

Change-Id: I022a39493756a3b5d1ed626fd05e74af3f043211
Reviewed-on: https://chromium-review.googlesource.com/410066
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>