Create PDFViewer.postMessage_().

Refactor usages of this.plugin_.postMessage() to use this method.

This makes it easier to pass a callback to other objects that sends
a message to the PPAPI plugin.

Bug: 55776,535978,748852
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I7ccd7df56f55d0ef099f590ba9e13c8bff9e3cfb
Reviewed-on: https://chromium-review.googlesource.com/907232
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535503}
4 files changed