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EvalJs has the following improvements over ExecuteScript:
 - One function that works for every return type
   (no AndExtractBool etc variants) and every fancy
   option (isolated worlds, user gestures).
 - No out-params; no out-param variables.
 - |script| doesn't need to call domAutomationController explicitly
   to return a value. Instead, it has eval() semantics -- the statement
   completion value becomes the result.
 - The result of EvalJs can be used directly in EXPECT_EQ; e.g.::
   EXPECT_EQ("about:blank", EvalJs(contents, "window.location.href"));
 - JS exceptions are reliably captured and will appear as C++ assertion
   failures.
 - JS stack traces arising from exceptions are annotated with the
   corresponding source code; this also appears in C++ assertion failures.
 - When a script doesn't produce a result, tests are much less likely
   to hang.
 - Doesn't get confused by crosstalk with other callers
   of domAutomationController.send().
 - Lists, dicts, null values, etc. can be returned as base::Values.

Limitations/warts:
 - The reliance on eval() currently means that a CSP script-src
   directive can interfere with the script runner. There is
   a workaround (use an isolated world), but this limitation
   currently prevents ExecJs from being a drop-in replacement
   for ExecuteScript. Fixing that will be a follow-on.

Change-Id: I260595094f71e7734d29a78e9ffd90b409d74f37
Reviewed-on: https://chromium-review.googlesource.com/1125121
Commit-Queue: Nick Carter <nick@chromium.org>
Reviewed-by: Ben Greenstein <bengr@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580702}