Tag filtering can cause line charts to be stale

[stephanwlee]

Ever since the data-loader-behavior refactor, the tf-line-chart-data-loader may not load based on the latest data requirement when:

1. tag filter to show plots of the same set of runs
2. examine the first and the second tags. Make sure you remember the general shape or values of two plots
3. type a character to match the second tag but not the first one. does the first chart have the correct values?

Cause:

tensorboard/tensorboard/components/tf_line_chart_data_loader/data-loader-behavior.ts

Line 83 in 4ee5780

'_loadKeyChanged(loadKey)',

does not get triggered even if the loadKey changes.

[stephanwlee]

#1492 also mitigates the issue because it correctly applies dom-repeat based on tag/run pair.