Log::MessageBuilder was already escaping most unsafe characters when
they were being logged, but plain backslashes were not. Merely updating
the existing escaping path was not sufficient, as recursion would cause
escape codes to be doubly escaped. This patches refactors the API to
ensure incoming text is escaped exactly once.

Bug: v8:8039
Change-Id: Id48aabf29fb6153189ae4a1ad7dfaaf4b41b62ad
Reviewed-on: https://chromium-review.googlesource.com/1169049Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55038}