Fix WebGLDebugShaders::getTranslatedShaderSource returning null

The function should return an empty string in case no source has been
defined, compileShader() has not been called, or the translation has
failed for shader. It used to return null, after this patch it returns an
empty string like it is supposed to.

Also make getTranslatedShaderSourceANGLE pure virtual in
WebGraphicsContext3D.

BUG=358530
TEST=WebGL conformance tests with --enable-privileged-webgl-extensions
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/216603004

git-svn-id: svn://svn.chromium.org/blink/trunk@170645 bbb929c8-8fbe-4397-9dbb-9b2b20218538