Fix advanced tab gas chart (#7380)

The gas chart on the advanced tab was not converting the gas price
selected into hex before setting it in state, resulting in the UI
throwing errors and the price being set incorrectly. It now converts
in the same manner as the input fields.

ui/app/components/app/gas-customization/gas-modal-page-container/advanced-tab-content/advanced-tab-content.component.js

@@ -1,5 +1,8 @@
 import React, { Component } from 'react'
 import PropTypes from 'prop-types'
+import {
+  decGWEIToHexWEI,
+} from '../../../../../helpers/utils/conversions.util'
 import Loading from '../../../../ui/loading-screen'
 import GasPriceChart from '../../gas-price-chart'
 import AdvancedGasInputs from '../../advanced-gas-inputs'
@@ -42,6 +45,11 @@ export default class AdvancedTabContent extends Component {
     )
   }
 
+  onGasChartUpdate = (price) => {
+    const { updateCustomGasPrice } = this.props
+    updateCustomGasPrice(decGWEIToHexWEI(price))
+  }
+
   render () {
     const { t } = this.context
     const {
@@ -78,7 +86,7 @@ export default class AdvancedTabContent extends Component {
             ? <div>
               <div className="advanced-tab__fee-chart__title">{ t('liveGasPricePredictions') }</div>
               {!gasEstimatesLoading
-                ? <GasPriceChart {...gasChartProps} updateCustomGasPrice={updateCustomGasPrice} />
+                ? <GasPriceChart {...gasChartProps} updateCustomGasPrice={this.onGasChartUpdate} />
                 : <Loading />
               }
               <div className="advanced-tab__fee-chart__speed-buttons">