Updates the ICU patch "double-conversion.diff" to only use the in-tree copy of
double-conversion when building Firefox/SpiderMonkey. Standalone ICU builds
instead use the ICU copy of double-conversion. Standalone ICU builds happen
when running "intl/icu_sources_data.py" and when converting the in-tree
little-endian ICU data file to the big-endian format.
`JS_HAS_INTL_API` is used to detect how ICU is built: When building ICU as part
of Firefox/SpiderMonkey, `JS_HAS_INTL_API` is guaranteed to be set, whereas in
standalone ICU builds `JS_HAS_INTL_API` isn't defined.
After applying the updated "double-conversion.diff" patch, ICU was reimported
to restore the previously deleted double-conversion sources files from ICU.
Depends on D190791
Differential Revision: https://phabricator.services.mozilla.com/D190792
a807279eb6