BAGHDAD — U.S. Secretary of State John Kerry backed Iraq’s prime minister on Friday in his efforts to resolve a mounting political crisis, underlining the importance of securing a “unified and functioning government” in the fight against the Islamic State. Kerry also pledged $155 million in new U.S. aid to Iraq. Making an unannounced visit to Baghdad, his first in two years, Kerry held discussions with Prime Minister Haider al-Abadi as well as with Iraq’s foreign minister, the Sunni speaker of parliament and a Kurdish regional leader in what was designed as a show of support for the Iraqi government as it struggles with ongoing security, economic and political challenges.