Refactor PendingScript / HTMLScriptRunner.

Goal: PendingScript should know how to watch for its resource to load, and upper
layers (such as HTMLScriptRunner) should delegate the "how to watch for load"
logic to PendingScript, instead of accessing its resource directly.

In addition, a function to extract source code from PendingScript should be in
PendingScript, not in HTMLScriptRunner.

The refactoring is split off from https://codereview.chromium.org/368283002
(script streaming), but it makes sense as standalone too.

BUG=

Review URL: https://codereview.chromium.org/551293002

git-svn-id: svn://svn.chromium.org/blink/trunk@181627 bbb929c8-8fbe-4397-9dbb-9b2b20218538