Chrome Version: 58.0.3029.110 (Official Build) (64-bit)
OS: OSX 10.11.6

What steps will reproduce the problem?
(1) install the attached tampermonkey script
(2) load any webpage
(3) open up the performance tab of dev tools
(4) click "Reload page" from the performance tab
(5) when the profiling is done, expand the "user timing" section

What is the expected result?
When you create a user-timing measure with neither `startMark` nor `endMark`, `navigationStart` is used as the inferred start time:
https://www.w3.org/TR/user-timing/#dom-performance-measure

Therefore, the left edge of the "m1" measure should be 0 in the timeline. 

What happens instead?
I'm consistently seeing the left edge line up with between 35 and 65 ms. 

Also, and possibly related, the red "load event" vertical line is suspiciously off by _about_ the same amount. In the example run from the screenshot, `performance.now()` in my `window.onload` handler was 3857.225. You can see that both items are ~110ms off.
 

Chrome Version: 58.0.3029.110 (Official Build) (64-bit)
OS: OSX 10.11.6

What steps will reproduce the problem?
(1) install the attached tampermonkey script
(2) load any webpage
(3) open up the performance tab of dev tools
(4) click "Reload page" from the performance tab
(5) when the profiling is done, expand the "user timing" section

What is the expected result?
1) 
When you create a user-timing measure with neither `startMark` nor `endMark`, `navigationStart` is used as the inferred start time:
https://www.w3.org/TR/user-timing/#dom-performance-measure

Therefore, the left edge of the "m1" measure should be 0 in the timeline. 

2) 
The value from `performance.now()` during and `onload` callback should be approx the same as the red "load event" vertical line in devtools

What happens instead?
1) 
In "m1 measure.png", the left edge of "m1" is ~26 ms.

2) 
In "onload indicator.png", they are ~34 ms apart. 
 

Chrome Version: 58.0.3029.110 (Official Build) (64-bit)
OS: OSX 10.11.6

What steps will reproduce the problem?
(1) install the attached tampermonkey script
(2) load any webpage
(3) open up the performance tab of dev tools
(4) click "Reload page" from the performance tab
(5) when the profiling is done, expand the "user timing" section

What is the expected result?
1) 
When you create a user-timing measure with neither `startMark` nor `endMark`, `navigationStart` is used as the inferred start time:
https://www.w3.org/TR/user-timing/#dom-performance-measure

Therefore, the left edge of the "m1" measure should be 0 in the timeline. 

2) 
The value from `performance.now()` during an `onload` callback should be approx the same as the red "load event" vertical line in devtools

What happens instead?
1) 
In "m1 measure.png", the left edge of "m1" is ~26 ms.

2) 
In "onload indicator.png", they are ~34 ms apart.