TextBlob: Paint simple text with text blobs.

This allows simple text to be painted with text blobs
on the HarfBuzz port (there are still differences in how
font platform data is applied to paints that makes it
convenient to start with one platform).

No caching is added by this CL, so turning on this feature
is a record performance regression (about 6% according to
cluster telemetry; higher on more text-heavy pages).
We hope to drive this down by improving the implementation
and by caching these blobs where possible.

This is guarded by a new RuntimeEnabledFeature.

BUG=377845

Review URL: https://codereview.chromium.org/509653002

git-svn-id: svn://svn.chromium.org/blink/trunk@181133 bbb929c8-8fbe-4397-9dbb-9b2b20218538