Expose --site-per-process in chrome://flags

BUG=99379

Review URL: https://codereview.chromium.org/880093002

Cr-Commit-Position: refs/heads/master@{#313385}

chrome/app/generated_resources.grd

@@ -14323,6 +14323,16 @@ Do you accept?
         Enable Web MIDI API experimental support.
       </message>
 
+      <!-- Site per process mode -->
+      <message name="IDS_FLAGS_ENABLE_SITE_PER_PROCESS_NAME"
+               desc="Name of about:flag option to turn on experimental out-of-process iframe support">
+        Enable out of process iframes
+      </message>
+      <message name="IDS_FLAGS_ENABLE_SITE_PER_PROCESS_DESCRIPTION"
+               desc="Description of about:flag option to turn on experimental out-of-process iframe support">
+        Highly experimental support for rendering cross-site iframes in separate processes.
+      </message>
+
       <!-- Windows 8 Metro mode. -->
       <if expr="is_win">
         <message name="IDS_METRO_FLOW_TAB_TITLE"

chrome/browser/about_flags.cc

@@ -2151,6 +2151,14 @@ const Experiment kExperiments[] = {
      MULTI_VALUE_TYPE(kMarkNonSecureAsChoices)
   },
 
+  {
+    "enable-site-per-process",
+    IDS_FLAGS_ENABLE_SITE_PER_PROCESS_NAME,
+    IDS_FLAGS_ENABLE_SITE_PER_PROCESS_DESCRIPTION,
+    kOsAll,
+    SINGLE_VALUE_TYPE(switches::kSitePerProcess)
+  },
+
   // NOTE: Adding new command-line switches requires adding corresponding
   // entries to enum "LoginCustomFlags" in histograms.xml. See note in
   // histograms.xml and don't forget to run AboutFlagsHistogramTest unit test.

tools/metrics/histograms/histograms.xml

@@ -50583,6 +50583,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="-836123854" label="wallet-service-use-sandbox"/>
   <int value="-820041355" label="enable-transition-compositing"/>
   <int value="-814097014" label="disable-session-crashed-bubble"/>
+  <int value="-813474479" label="site-per-process"/>
   <int value="-795600188" label="disable-async-dns"/>
   <int value="-780798969" label="disable-single-click-autofill"/>
   <int value="-770319039" label="enable-touch-editing"/>