code: function _getGradientTexture() {
          // Only update the texture when the gradient has changed.
          if (this._prevGradientColors === this.props.gradientColors) {
              return this._gradientTexture;
          }
          var canvas = document.createElement('canvas');
          // 512, 10 because these are the same dimensions webgl-heatmap uses for its
          // built in gradient textures.
          var width = 512;
          var height = 10;
          canvas.width = String(width);
          canvas.height = String(height);
          var ctx = canvas.getContext('2d');
          var gradient = ctx.createLinearGradient(0, height / 2, width, height / 2);
          var colors = this.props.gradientColors;
          colors.forEach(function each(color, index) {
              var position = index / (colors.size - 1);
              gradient.addColorStop(position, color);
          });
          ctx.fillStyle = gradient;
          ctx.fillRect(0, 0, width, height);
          var image = new window.Image();
          image.src = canvas.toDataURL('image/png');
          return image;
      }
docs: Updates `this._gradientTexture` Image if `props.gradientColors`
  has changed.
  @returns {Image} `this._gradientTexture`.