[spelling] existance to existence

This CL corrects the spelling of existence.

(code health)
BUG=None
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2916723002
Cr-Original-Commit-Position: refs/heads/master@{#478403}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 118577369265f078d8f4e6d7ad7b99833aa833a1
diff --git a/tools/gn/docs/reference.md b/tools/gn/docs/reference.md
index 735e5f9..2e6aa6f 100644
--- a/tools/gn/docs/reference.md
+++ b/tools/gn/docs/reference.md
@@ -5111,7 +5111,7 @@
   For binary targets (source sets, executables, and libraries), the known file
   types will be compiled with the associated tools. Unknown file types and
   headers will be skipped. However, you should still list all C/C+ header files
-  so GN knows about the existance of those files for the purposes of include
+  so GN knows about the existence of those files for the purposes of include
   checking.
 
   As a special case, a file ending in ".def" will be treated as a Windows
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index d304bc5..bbb8791 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -1715,7 +1715,7 @@
   For binary targets (source sets, executables, and libraries), the known file
   types will be compiled with the associated tools. Unknown file types and
   headers will be skipped. However, you should still list all C/C+ header files
-  so GN knows about the existance of those files for the purposes of include
+  so GN knows about the existence of those files for the purposes of include
   checking.
 
   As a special case, a file ending in ".def" will be treated as a Windows