High memory usage of the browser process after some time (leak?)

[atuchin-m]

Steps (very unclear for now):

1. Use a browser in a normal way from sometime;
2. Close all the tab except about:blank;
3. Measure the browser process memory usage using internal task manager;
4. Restart the browser (with about:blank) and measure again.

Expectation:
Memory usage in 3) should be <= 1.5x of the amount in 4)

Actual:
385MB in 3), less than 100mb in 4)

[atuchin-m]

One of possible reason is #21603 (comment).
Looks like we need to measure memory used by Rust somehow.
Here is Mozilla approach: https://github.com/servo/servo/blob/master/components/malloc_size_of/lib.rs

[atuchin-m]

I'm able to reproduce on a fresh profile after opening 5 heavy sites. The screenshots with/without adblock (was disabled by deleting and blocking the component downloading).

So the adblock is a primary reason for this.

[iefremov]

First step would be #26892