[unseasoned-pdf] Queue postMessage() before 'connect'

Delays calls to the unseasoned plugin's postMessage() method before the
initial 'connect' message from the plugin. Otherwise, these calls will
fail due to the undefined postMessage() method.

Also refactors handleScriptingMessage() to avoid a duplicate 'connect'
message caused by confusing the 'connect' message with a scripting
message from the embedder.

Bug: 1228987
Change-Id: Ib8bc1f55e06bc69d7777fc6b68ff0b07a3a040d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3200186
Auto-Submit: K. Moon <kmoon@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#927527}