[ServiceWorker] Fix jumbo+Win compilation error related to 'PostMessage'

PostMessage is a macro in Windows and depending on whether Windows
headers have been included or not, PostMessage will either be treated as
PostMessage or PostMessageW.

However, our previous CL
introduced a method just named as 'PostMessage', this caused some
compile/link errors on Windows platform.

This CL renames the method to 'PostMessageToServiceWorker' to fix this.

TBR=kinuko@chromium.org for mechanical renaming in the mojom file

Change-Id: I16241052e3deb8ba4a5fbb777037eca9904c6279
Reviewed-on: https://chromium-review.googlesource.com/959688
Commit-Queue: Han Leon <leon.han@intel.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542741}
9 files changed