Fix textLength position adjustment for the "adjust spacing" case

third_party/WebKit/LayoutTests/svg/text/select-textLength-spacing-stretch-4-expected.txt

-layer at (0,0) size 800x600
-  LayoutView at (0,0) size 800x600
-layer at (0,0) size 800x600
-  LayoutSVGRoot {svg} at (0,0) size 800x600
-    LayoutSVGContainer {g} at (10,1) size 144.26x11.20
-      LayoutSVGPath {svg:line} at (10,1) size 0x11.20 [stroke={[type=SOLID] [color=#008000]}] [fill={[type=SOLID] [color=#000000]}] [x1=10.00] [y1=1.00] [x2=10.00] [y2=12.20]
-      LayoutSVGPath {svg:line} at (154.26,1) size 0x11.20 [stroke={[type=SOLID] [color=#008000]}] [fill={[type=SOLID] [color=#000000]}] [x1=154.26] [y1=1.00] [x2=154.26] [y2=12.20]
-      LayoutSVGRect {svg:rect} at (10,1) size 144.26x11.20 [fill={[type=SOLID] [color=#FF0000] [opacity=0.40]}] [x=10.00] [y=1.00] [width=144.26] [height=11.20]
-    LayoutSVGText {text} at (10,1) size 144.23x11.19 contains 1 chunk(s)
-      LayoutSVGInlineText {#text} at (10,1) size 144.23x11.19
-        chunk 1 text run 1 at (10.00,10.00) startOffset 0 endOffset 1 width 6.11: "T"
-        chunk 1 text run 2 at (20.74,10.00) startOffset 1 endOffset 2 width 5.56: "e"
-        chunk 1 text run 3 at (32.04,10.00) startOffset 2 endOffset 3 width 5.00: "x"
-        chunk 1 text run 4 at (42.78,10.00) startOffset 3 endOffset 4 width 2.78: "t"
-        chunk 1 text run 5 at (51.30,10.00) startOffset 4 endOffset 5 width 2.78: " "
-        chunk 1 text run 6 at (59.81,10.00) startOffset 5 endOffset 6 width 2.78: "t"
-        chunk 1 text run 7 at (68.33,10.00) startOffset 6 endOffset 7 width 5.56: "o"
-        chunk 1 text run 8 at (79.63,10.00) startOffset 7 endOffset 8 width 2.78: " "
-        chunk 1 text run 9 at (88.15,10.00) startOffset 8 endOffset 9 width 6.67: "S"
-        chunk 1 text run 10 at (100.56,10.00) startOffset 9 endOffset 10 width 2.78: "t"
-        chunk 1 text run 11 at (109.07,10.00) startOffset 10 endOffset 11 width 3.33: "r"
-        chunk 1 text run 12 at (118.14,10.00) startOffset 11 endOffset 12 width 5.56: "e"
-        chunk 1 text run 13 at (129.44,10.00) startOffset 12 endOffset 13 width 2.78: "t"
-        chunk 1 text run 14 at (137.96,10.00) startOffset 13 endOffset 14 width 5.00: "c"
-        chunk 1 text run 15 at (148.70,10.00) startOffset 14 endOffset 15 width 5.56: "h"
-selection start: position 0 of child 0 {#text} of child 5 {text} of child 0 {svg} of document
-selection end:   position 15 of child 0 {#text} of child 5 {text} of child 0 {svg} of document