Don't strip rel=preload hints for resource types we don't optimize

[jeffkaufman]

Right now if we see:

<link rel=preload as=font href=...>

or:

<link rel=preload as=document href=...>

we will strip the preload hint. We should instead only be stripping hints for resources we optimize:

<link rel=preload as=script href=...>
<link rel=preload as=style href=...>
<link rel=preload as=image href=...>

Specifically, if as is something other than script, style, or image we should leave the hint alone.

[jmarantz]

And we should only strip preload hints for images when image-rewriting is on, and options()->image_preserve_urls() is off.

Similar for css_preserve_urls() and the variety of css-rewriting options (mostly combine_css and rewrite_css and cache_extend_styles).

Similar for js_preserve_urls() and js-rewriting options including combine_javascript and rewrite_external_javascript and cache_extend_javascript but not rewrite_inline_javascript.

We'll have to be a little careful about curating the exact list of filters that trigger the stripping.

[jeffkaufman]

Right now strip_subresource_hints_filter.cc enables hint removal if:

driver_->can_modify_urls() &&
(!(options->css_preserve_urls() &&
   options->image_preserve_urls() &&
   options->js_preserve_urls()));

Each HtmlFilter has bool CanModifyUrls(), and driver_->can_modify_urls() just returns whether any enabled filter has CanModifyUrls() returning true``(SeeHtmlParse::CheckFilterBehavior`.)

This was written this way because at the time hints didn't specify types.

We can extend this to add CanModifyImageUrls(), CanModifySyleUrls(), CanModifyScriptUrls() but that's a much bigger chunk of work.

[jmarantz]

I might be kind of frustrated if I was using MPS just for aggressive image optimization, and it stripped all my CSS/JS hints. How much work is this really?

[jeffkaufman]

How much work is this really?

Kind of a pain. Right now RewriteFilter::CanModifyUrls just returns true, so we'd need to go through all our filter classes and specify what kinds of urls they modify.

Alternatively, if X_preserve_urls() is set we could not strip X hints, and so your example person could set preserve for css+js but not images. That one is much easier to code, since strip_subresource_hints_filter can see (and already uses) the preserve settings.

[jeffkaufman]

Planning to go the preserve route.

We can come back and do the full thing later if we want to.