The Harmony spec specifies two button types: Primary and Secondary. Primary buttons fill with blue (vs. white), and a secondary ui dialog can contain only one Primary button.
Before the Harmony spec there was a push to convert Chrome secondary UI to Material Design, which was the genesis of MdTextButton. That Material Design effort turned into Harmony, and so MdTextButton became the implementation behind Harmony buttons.
MdTextButton's blue button variant is called a "call to action" button. "Call to action" is an advertising term, not a UI term, so it should not have become part of the MdTextButton API (I think the Material Design changes were undertaken without a design doc). More importantly, if an engineer today is asked to implement a Harmony dialog that includes a Primary button, they must figure out that calling SetCallToAction(true) creates a Primary button. MdTextButton's API needs to be changed to match the Harmony spec, so that creating a Primary button is done with a straightforward and obvious call to SetPrimary(true).
Link to the Harmony sticker sheet showing the Primary and Secondary button styles:
https://folio.googleplex.com/chrome-ux-specs-and-sources/Chrome%20browser%20(MD)/Secondary%20UI%20Previews%20and%20specs%20(exports)#%2F_P%20-%20Chrome%20-%20stickersheet.png%3Fz=width
/Secondary%20UI%20Previews%20and%20specs%20(exports)#%2F_P%20-%20Chrome%20-%20stickersheet.png%3Fz=width){kind=link}
Comment 1 by est...@chromium.org
, Sep 9 2016