Effects that raise or lower a creature's power and/or toughness, such as the effect of Titanic Growth, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness and effects that switch its power and toughness.

If Chromium gains an ability after its activated ability resolves, it will keep that ability.

Chromium stops being an Elder Dragon for the turn once its last ability has resolved. It's still a legendary creature named Chromium, the Mutable.

Activating Chromium's last ability after it has become blocked won't cause Chromium to become unblocked.

Chromium's activated ability overwrites all previous effects that set its base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after that ability resolves will overwrite this effect.