TOKYO (Reuters) - Robots could fill the jobs of 3.5 million people in grayingJapan by 2025, a thinktank says, helping to avert worker shortages as the country's population shrinks. Japan faces a 16 percent slide in the size of its workforce by 2030 while the number of elderly will mushroom, the government estimates, raising worries about who will do the work in a country unused to, and unwilling to contemplate, large-scale immigration.