Allow flags strings in quotes.

Allow flags strings in quotes.

If the user wants to run Chromium and pass several js flags to V8, the command
line would be:
chrome --js-flags="--foo --bar"

In this case, the value of the flag is "--foo --bar" (including quotes; this is
conceptually correct), but V8 doesn't parse this properly.

This CL fixes SetFlagsFromString to handle quotes.

R=yangguo@chromium.org
BUG=

[marja, 2014-04-02 14:54:23]

yangguo, wdyt?

An alternative solution would be fix all callers, but I think this is more
correct.

[Yang, 2014-04-02 15:19:30]

On 2014/04/02 14:54:23, marja wrote:
> yangguo, wdyt?
> 
> An alternative solution would be fix all callers, but I think this is more
> correct.

lgtm.