[blink] Harden inspector ScriptResource content access

Check for an edge case where a ScriptResource may have a null resource
buffer without having been loaded. As part of this, extract the
inspector's logic for accessing a ScriptResource's text into the
ScriptResource, as it relies on ScriptResource internal state.

Bug: 865098
Bug: 899217
Change-Id: I5ceb6f105e21e4eeb7ce5f9bc82c7d7c7f7f6aad
Reviewed-on: https://chromium-review.googlesource.com/c/1352183
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611255}
3 files changed