Fix process reuse for dedicated processes when over process limit.

content/public/browser/render_process_host.h

Index: content/public/browser/render_process_host.h
diff --git a/content/public/browser/render_process_host.h b/content/public/browser/render_process_host.h
index 97595f18ea206f1836685b20686f878185384eeb..03c9bb026e68c4f6c6c1143a7bf75452ecbf8425 100644
--- a/content/public/browser/render_process_host.h
+++ b/content/public/browser/render_process_host.h
@@ -352,6 +352,13 @@ class CONTENT_EXPORT RenderProcessHost : public IPC::Sender,
   virtual void SetIsNeverSuitableForReuse() = 0;
   virtual bool MayReuseHost() = 0;
 
+  // Indicates whether this RenderProcessHost can be used to host a URL that
+  // requires a dedicated process.  This starts out as true for new processes
+  // and will become false after a call to UnsetCanBecomeDedicatedProcess(),
+  // such as after the process has been used to host web content.
+  virtual bool CanBecomeDedicatedProcess() = 0;

[alexmos, 2017/06/12 17:35:07]

These are public because of MockRenderProcessHost. :(  I struggled with finding
good names here.  What I really wanted this to mean was that this is true up
until the first LockToOrigin call (which might come either when the first
navigation commits, or when a SiteInstance with a site that's already set calls
LockToOrigin).  I.e., this intends to be true for
uninitialized/untainted/site-less SiteInstances so that IsSuitableHost allows
their process to be used as a dedicated process.  Without this, the very first
navigation in a new tab caused a process swap from the initial SiteInstance,
causing lots of test failures.

[Charlie Reis, 2017/06/12 23:08:17]

Sigh.  Some day we'll fix that.

[Charlie Reis, 2017/06/12 23:08:17]

Yeah, I wonder if there's a better concept to be tracking here.  Would something
like IsUnused work, with documentation that it gets cleared in any of the
following cases?
1) Commits a page.
2) Given to a SiteInstance that already has a site assigned.
3) Hosts a ServiceWorker or SharedWorker.

That makes it potentially more generally useful, without being specific to the
dedicated process case.

[alexmos, 2017/06/14 22:39:05]

I like this, especially given the potential for this to be used elsewhere. 
Changed.

(Note that I didn't add (3) to the comment - see my explanation in
https://codereview.chromium.org/2921063003/diff/80001/content/browser/rendere....
 Though I guess I could say that ServiceWorkers are implicitly handled via (2),
and SharedWorkers are implicitly handled via (1).)

[Charlie Reis, 2017/06/17 23:13:53]

Yes, saying it's implicit in those cases might be worthwhile, just to show that
those cases are covered.

[alexmos, 2017/06/19 20:03:58]

Done.

+  virtual void UnsetCanBecomeDedicatedProcess() = 0;
+
   // Returns the current number of active views in this process.  Excludes
   // any RenderViewHosts that are swapped out.
   size_t GetActiveViewCount();