Simplify AST transformations that need to find main Share code for finding the main function from the AST between InitializeVariables, DeferGlobalInitializers, EmulateGLFragColorBroadcast and UseInterfaceBlockFields. This makes InitializeVariables simpler in particular, as it doesn't need an AST traverser anymore. BUG=angleproject:2033 TEST=angle_unittests, WebGL conformance tests Change-Id: I14c994bbde58a904f6684d2f0b72bd8004f70902 Reviewed-on: https://chromium-review.googlesource.com/501166 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>