This CL improves break locations for expressions like 'var a = <expr>'. Without CL we use <expr> position as break location for initialization statement, with this CL we use position of first character after '=' as position.
Benefits (see test for details):
- only one break in expressions which includes mix of property lookup and calls, e.g. var p = Promise.resolve().then(x => x * 2),
- removed redundant break location for expressions like: let { x, y } = { x: 1, y: 2}.

Bug: v8:5909
Change-Id: I039d911903a2826c9859710a63ab0462c992e11b
Reviewed-on: https://chromium-review.googlesource.com/513926
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45530}