DevTools: Have CodeMirror observe default indentation value.
DevTools: Have CodeMirror observe default indentation value.

CodeMirror currently gets handed a default indent size of 4 spaces, and the
indentation setting is ignored. This patch updates the CodeMirror config to use
the value in the settings, and observes changes to its value.

Bug: 977394
Change-Id: I1ddaeaa788378e37a88da1d7f42659bbe386dd57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674092
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Commit-Queue: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@google.com>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#696792}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d3be8efb8e3700d72b6a75e1078b54e2b17e8c15

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